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11.13 Using the FPU
Strangely enough, most of assembly language literature does not even mention the existence of the FPU, or floating point unit, let alone discuss programming it.
Yet, never does assembly language shine more than when we create highly optimized FPU code by doing things that can be done only in assembly language.
11.13.1 Organization of the FPU
The FPU consists of 8 80-bit floating-point
registers. These are organized in a stack fashion--you can push a value on TOS (top of stack) and you can pop it.
That said, the assembly language op codes are not push
and pop because those are already taken.
You can push a value on TOS by using fld, fild, and fbld. Several other op
codes let you push many common constants--such as pi--on the TOS.
Similarly, you can pop a value by using fst, fstp, fist, fistp, and fbstp. Actually, only the op codes that end with a p will literally pop the value, the rest will store
it somewhere else without removing it from the TOS.
We can transfer the data between the TOS and the computer memory either as a 32-bit, 64-bit, or 80-bit real, a 16-bit, 32-bit, or 64-bit integer, or an 80-bit packed decimal.
The 80-bit packed decimal is a special case of binary coded decimal which is very convenient when converting between the ASCII representation of data and the internal data of the FPU. It allows us to use 18 significant digits.
No matter how we represent data in the memory, the FPU always stores it in the 80-bit real format in its registers.
Its internal precision is at least 19 decimal digits, so even if we choose to display results as ASCII in the full 18-digit precision, we are still showing correct results.
We can perform mathematical operations on the TOS: We can calculate its sine, we can scale it (i.e., we can multiply or divide it by a power of 2), we can calculate its base-2 logarithm, and many other things.
We can also multiply or divide it by, add it to, or subtract it from, any of the FPU registers (including itself).
The official Intel op code for the TOS is st, and for the registers st(0)-st(7). st and st(0), then, refer to the same register.
For whatever reasons, the original author of nasm has
decided to use different op codes, namely st0-st7. In other words, there are no parentheses, and the TOS is always st0, never just st.
11.13.1.1 The Packed Decimal Format
The packed decimal format uses 10 bytes (80 bits) of memory to represent 18 digits. The number represented there is always an integer.
Tip: You can use it to get decimal places by multiplying the TOS by a power of 10 first.
The highest bit of the highest byte (byte 9) is the sign bit: If it is set, the number is negative, otherwise, it is positive. The rest of the bits of this byte are unused/ignored.
The remaining 9 bytes store the 18 digits of the number: 2 digits per byte.
The more significant digit is stored in the high nibble (4 bits), the less significant digit in the low nibble.
That said, you might think that -1234567 would be stored
in the memory like this (using hexadecimal notation):
80 00 00 00 00 00 01 23 45 67
Alas it is not! As with everything else of Intel make, even the packed decimal is little-endian.
That means our -1234567 is stored like this:
67 45 23 01 00 00 00 00 00 80
Remember that, or you will be pulling your hair out in desperation!
Note: The book to read--if you can find it--is Richard Startz' 8087/80287/80387 for the IBM PC & Compatibles. Though it does seem to take the fact about the little-endian storage of the packed decimal for granted. I kid you not about the desperation of trying to figure out what was wrong with the filter I show below before it occurred to me I should try the little-endian order even for this type of data.
11.13.2 Excursion to Pinhole Photography
To write meaningful software, we must not only understand our programming tools, but also the field we are creating software for.
Our next filter will help us whenever we want to build a pinhole camera, so, we need some background in pinhole photography before we can continue.
11.13.2.1 The Camera
The easiest way to describe any camera ever built is as some empty space enclosed in some lightproof material, with a small hole in the enclosure.
The enclosure is usually sturdy (e.g., a box), though sometimes it is flexible (the bellows). It is quite dark inside the camera. However, the hole lets light rays in through a single point (though in some cases there may be several). These light rays form an image, a representation of whatever is outside the camera, in front of the hole.
If some light sensitive material (such as film) is placed inside the camera, it can capture the image.
The hole often contains a lens, or a lens assembly, often called the objective.
11.13.2.2 The Pinhole
But, strictly speaking, the lens is not necessary: The original cameras did not use a lens but a pinhole. Even today, pinholes are used, both as a tool to study how cameras work, and to achieve a special kind of image.
The image produced by the pinhole is all equally sharp. Or blurred. There is an ideal size for a pinhole: If it is either larger or smaller, the image loses its sharpness.
11.13.2.3 Focal Length
This ideal pinhole diameter is a function of the square root of focal length, which is the distance of the pinhole from the film.
D = PC * sqrt(FL)
In here, D is the ideal diameter of the pinhole, FL is the focal length, and PC is a
pinhole constant. According to Jay Bender, its value is 0.04, while Kenneth Connors has determined it to be 0.037. Others have proposed other values. Plus, this value is for
the daylight only: Other types of light will require a different constant, whose value
can only be determined by experimentation.
11.13.2.4 The F-Number
The f-number is a very useful measure of how much light reaches the film. A light meter can determine that, for example, to expose a film of specific sensitivity with f5.6 may require the exposure to last 1/1000 sec.
It does not matter whether it is a 35-mm camera, or a 6x9cm camera, etc. As long as we know the f-number, we can determine the proper exposure.
The f-number is easy to calculate:
F = FL / D
In other words, the f-number equals the focal length divided by the diameter of the pinhole. It also means a higher f-number either implies a smaller pinhole or a larger focal distance, or both. That, in turn, implies, the higher the f-number, the longer the exposure has to be.
Furthermore, while pinhole diameter and focal distance are one-dimensional measurements, both, the film and the pinhole, are two-dimensional. That means that if you have measured the exposure at f-number A as t, then the exposure at f-number B is:
t * (B / A)²
11.13.2.5 Normalized F-Number
While many modern cameras can change the diameter of their pinhole, and thus their f-number, quite smoothly and gradually, such was not always the case.
To allow for different f-numbers, cameras typically contained a metal plate with several holes of different sizes drilled to them.
Their sizes were chosen according to the above formula in such a way that the resultant f-number was one of standard f-numbers used on all cameras everywhere. For example, a very old Kodak Duaflex IV camera in my possession has three such holes for f-numbers 8, 11, and 16.
A more recently made camera may offer f-numbers of 2.8, 4, 5.6, 8, 11, 16, 22, and 32 (as well as others). These numbers were not chosen arbitrarily: They all are powers of the square root of 2, though they may be rounded somewhat.
11.13.2.6 The F-Stop
A typical camera is designed in such a way that setting any of the normalized f-numbers changes the feel of the dial. It will naturally stop in that position. Because of that, these positions of the dial are called f-stops.
Since the f-numbers at each stop are powers of the square root of 2, moving the dial by 1 stop will double the amount of light required for proper exposure. Moving it by 2 stops will quadruple the required exposure. Moving the dial by 3 stops will require the increase in exposure 8 times, etc.
11.13.3 Designing the Pinhole Software
We are now ready to decide what exactly we want our pinhole software to do.
11.13.3.1 Processing Program Input
Since its main purpose is to help us design a working pinhole camera, we will use the focal length as the input to the program. This is something we can determine without software: Proper focal length is determined by the size of the film and by the need to shoot "regular" pictures, wide angle pictures, or telephoto pictures.
Most of the programs we have written so far worked with individual characters, or bytes, as their input: The hex program converted individual bytes into a hexadecimal number, the csv program either let a character through, or deleted it, or changed it to a different character, etc.
One program, ftuc used the state machine to consider at most two input bytes at a time.
But our pinhole program cannot just work with individual characters, it has to deal with larger syntactic units.
For example, if we want the program to calculate the pinhole diameter (and other
values we will discuss later) at the focal lengths of 100
mm, 150 mm, and 210
mm, we may want to enter something like this:
100, 150, 210
Our program needs to consider more than a single byte of input at a time. When it sees
the first 1, it must understand it is seeing the first
digit of a decimal number. When it sees the 0 and the other
0, it must know it is seeing more digits of the same
number.
When it encounters the first comma, it must know it is no longer receiving the digits
of the first number. It must be able to convert the digits of the first number into the
value of 100. And the digits of the second number into the
value of 150. And, of course, the digits of the third
number into the numeric value of 210.
We need to decide what delimiters to accept: Do the input numbers have to be separated by a comma? If so, how do we treat two numbers separated by something else?
Personally, I like to keep it simple. Something either is a number, so I process it. Or it is not a number, so I discard it. I do not like the computer complaining about me typing in an extra character when it is obvious that it is an extra character. Duh!
Plus, it allows me to break up the monotony of computing and type in a query instead of just a number:
What is the best pinhole diameter for the focal length of 150?
There is no reason for the computer to spit out a number of complaints:
Syntax error: What Syntax error: is Syntax error: the Syntax error: best
Et cetera, et cetera, et cetera.
Secondly, I like the # character to denote the start of
a comment which extends to the end of the line. This does not take too much effort to
code, and lets me treat input files for my software as executable scripts.
In our case, we also need to decide what units the input should come in: We choose millimeters because that is how most photographers measure the focus length.
Finally, we need to decide whether to allow the use of the decimal point (in which case we must also consider the fact that much of the world uses a decimal comma).
In our case allowing for the decimal point/comma would offer a false sense of
precision: There is little if any noticeable difference between the focus lengths of
50 and 51, so allowing the
user to input something like 50.5 is not a good idea. This
is my opinion, mind you, but I am the one writing this program. You can make other
choices in yours, of course.
11.13.3.2 Offering Options
The most important thing we need to know when building a pinhole camera is the
diameter of the pinhole. Since we want to shoot sharp images, we will use the above
formula to calculate the pinhole diameter from focal length. As experts are offering
several different values for the PC constant, we will need
to have the choice.
It is traditional in UNIX® programming to have two main ways of choosing program parameters, plus to have a default for the time the user does not make a choice.
Why have two ways of choosing?
One is to allow a (relatively) permanent choice that applies automatically each time the software is run without us having to tell it over and over what we want it to do.
The permanent choices may be stored in a configuration file, typically found in the user's home directory. The file usually has the same name as the application but is started with a dot. Often "rc" is added to the file name. So, ours could be ~/.pinhole or ~/.pinholerc. (The ~/ means current user's home directory.)
The configuration file is used mostly by programs that have many configurable parameters. Those that have only one (or a few) often use a different method: They expect to find the parameter in an environment variable. In our case, we might look at an environment variable named PINHOLE.
Usually, a program uses one or the other of the above methods. Otherwise, if a configuration file said one thing, but an environment variable another, the program might get confused (or just too complicated).
Because we only need to choose one such parameter, we will go with the second method and search the environment for a variable named PINHOLE.
The other way allows us to make ad hoc decisions: "Though I usually want you to use 0.039, this time I want 0.03872." In other words, it allows us to override the permanent choice.
This type of choice is usually done with command line parameters.
Finally, a program always needs a default. The user may not make any choices. Perhaps he does not know what to choose. Perhaps he is "just browsing." Preferably, the default will be the value most users would choose anyway. That way they do not need to choose. Or, rather, they can choose the default without an additional effort.
Given this system, the program may find conflicting options, and handle them this way:
-
If it finds an ad hoc choice (e.g., command line parameter), it should accept that choice. It must ignore any permanent choice and any default.
-
Otherwise, if it finds a permanent option (e.g., an environment variable), it should accept it, and ignore the default.
-
Otherwise, it should use the default.
We also need to decide what format our PC option should
have.
At first site, it seems obvious to use the PINHOLE=0.04 format for the environment variable, and -p0.04 for the command line.
Allowing that is actually a security risk. The PC
constant is a very small number. Naturally, we will test our software using various small
values of PC. But what will happen if someone runs the
program choosing a huge value?
It may crash the program because we have not designed it to handle huge numbers.
Or, we may spend more time on the program so it can handle huge numbers. We might do that if we were writing commercial software for computer illiterate audience.
Or, we might say, "Tough! The user should know better.""
Or, we just may make it impossible for the user to enter a huge number. This is the approach we will take: We will use an implied 0. prefix.
In other words, if the user wants 0.04, we will expect
him to type -p04, or set PINHOLE=04 in his environment. So, if he says -p9999999, we will interpret it as 0.9999999--still ridiculous but at least safer.
Secondly, many users will just want to go with either Bender's constant or Connors' constant. To make it easier on them, we will interpret -b as identical to -p04, and -c as identical to -p037.
11.13.3.3 The Output
We need to decide what we want our software to send to the output, and in what format.
Since our input allows for an unspecified number of focal length entries, it makes
sense to use a traditional database-style output of showing the result of the calculation
for each focal length on a separate line, while separating all values on one line by a
tab character.
Optionally, we should also allow the user to specify the use of the CSV format we have studied earlier. In this case, we will print
out a line of comma-separated names describing each field of every line, then show our
results as before, but substituting a comma for the tab.
We need a command line option for the CSV format. We cannot use -c because that already means use Connors' constant. For some strange reason, many web sites refer to CSV files as "Excel spreadsheet" (though the CSV format predates Excel). We will, therefore, use the -e switch to inform our software we want the output in the CSV format.
We will start each line of the output with the focal length. This may sound repetitious at first, especially in the interactive mode: The user types in the focal length, and we are repeating it.
But the user can type several focal lengths on one line. The input can also come in from a file or from the output of another program. In that case the user does not see the input at all.
By the same token, the output can go to a file which we will want to examine later, or it could go to the printer, or become the input of another program.
So, it makes perfect sense to start each line with the focal length as entered by the user.
No, wait! Not as entered by the user. What if the user types in something like this:
00000000150
Clearly, we need to strip those leading zeros.
So, we might consider reading the user input as is, converting it to binary inside the FPU, and printing it out from there.
But...
What if the user types something like this:
17459765723452353453534535353530530534563507309676764423
Ha! The packed decimal FPU format lets us input 18-digit numbers. But the user has entered more than 18 digits. How do we handle that?
Well, we could modify our code
to read the first 18 digits, enter it to the FPU, then
read more, multiply what we already have on the TOS by
10 raised to the number of additional digits, then add to
it.
Yes, we could do that. But in this program it would be ridiculous (in a different one it may be just the thing to do): Even the circumference of the Earth expressed in millimeters only takes 11 digits. Clearly, we cannot build a camera that large (not yet, anyway).
So, if the user enters such a huge number, he is either bored, or testing us, or trying to break into the system, or playing games--doing anything but designing a pinhole camera.
What will we do?
We will slap him in the face, in a manner of speaking:
17459765723452353453534535353530530534563507309676764423 ??? ??? ??? ??? ???
To achieve that, we will simply ignore any leading zeros. Once we find a non-zero
digit, we will initialize a counter to 0 and start taking
three steps:
-
Send the digit to the output.
-
Append the digit to a buffer we will use later to produce the packed decimal we can send to the FPU.
-
Increase the counter.
Now, while we are taking these three steps, we also need to watch out for one of two conditions:
-
If the counter grows above 18, we stop appending to the buffer. We continue reading the digits and sending them to the output.
-
If, or rather when, the next input character is not a digit, we are done inputting for now.
Incidentally, we can simply discard the non-digit, unless it is a
#, which we must return to the input stream. It starts a comment, so we must see it after we are done producing output and start looking for more input.
That still leaves one possibility uncovered: If all the user enters is a zero (or several zeros), we will never find a non-zero to display.
We can determine this has happened whenever our counter stays at 0. In that case we need to send 0
to the output, and perform another "slap in the face":
0 ??? ??? ??? ??? ???
Once we have displayed the focal length and determined it is valid (greater than 0 but not exceeding 18 digits), we can calculate the pinhole
diameter.
It is not by coincidence that pinhole contains the word pin. Indeed, many a pinhole literally is a pin hole, a hole carefully punched with the tip of a pin.
That is because a typical pinhole is very small. Our formula gets the result in
millimeters. We will multiply it by 1000, so we can output
the result in microns.
At this point we have yet another trap to face: Too much precision.
Yes, the FPU was designed for high precision mathematics. But we are not dealing with high precision mathematics. We are dealing with physics (optics, specifically).
Suppose we want to convert a truck into a pinhole camera (we would not be the first
ones to do that!). Suppose its box is 12 meters long, so we
have the focal length of 12000. Well, using Bender's
constant, it gives us square root of 12000 multiplied by
0.04, which is 4.381780460
millimeters, or 4381.780460 microns.
Put either way, the result is absurdly precise. Our truck is not exactly 12000 millimeters long. We did not measure its length with such a
precision, so stating we need a pinhole with the diameter of 4.381780460 millimeters is, well, deceiving. 4.4 millimeters would do just fine.
Note: I "only" used ten digits in the above example. Imagine the absurdity of going for all 18!
We need to limit the number of significant digits of our result. One way of doing it
is by using an integer representing microns. So, our truck would need a pinhole with the
diameter of 4382 microns. Looking at that number, we still
decide that 4400 microns, or 4.4 millimeters is close enough.
Additionally, we can decide that no matter how big a result we get, we only want to display four significant digits (or any other number of them, of course). Alas, the FPU does not offer rounding to a specific number of digits (after all, it does not view the numbers as decimal but as binary).
We, therefore, must devise an algorithm to reduce the number of significant digits.
Here is mine (I think it is awkward--if you know a better one, please, let me know):
-
Initialize a counter to
0. -
While the number is greater than or equal to
10000, divide it by10and increase the counter. -
Output the result.
-
While the counter is greater than
0, output0and decrease the counter.
Note: The
10000is only good if you want four significant digits. For any other number of significant digits, replace10000with10raised to the number of significant digits.
We will, then, output the pinhole diameter in microns, rounded off to four significant digits.
At this point, we know the focal length and the pinhole diameter. That means we have enough information to also calculate the f-number.
We will display the f-number, rounded to four significant digits. Chances are the f-number will tell us very little. To make it more meaningful, we can find the nearest normalized f-number, i.e., the nearest power of the square root of 2.
We do that by multiplying the actual f-number by itself, which, of course, will give
us its square. We will then calculate its base-2 logarithm,
which is much easier to do than calculating the base-square-root-of-2 logarithm! We will
round the result to the nearest integer. Next, we will raise 2 to the result. Actually,
the FPU gives us a good shortcut to do that: We can
use the fscale op code to "scale" 1, which is analogous to
shifting an integer left. Finally, we calculate the square
root of it all, and we have the nearest normalized f-number.
If all that sounds overwhelming--or too much work, perhaps--it may become much clearer if you see the code. It takes 9 op codes altogether:
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
The first line, fmul st0, st0, squares the contents of
the TOS (top of the stack, same as st, called st0 by nasm). The fld1 pushes 1 on the TOS.
The next line, fld st1, pushes the square back to the
TOS. At this point the square is both in st and st(2) (it will become clear why
we leave a second copy on the stack in a moment). st(1)
contains 1.
Next, fyl2x calculates base-2 logarithm of st multiplied by st(1). That is why we
placed 1 on st(1) before.
At this point, st contains the logarithm we have just calculated, st(1) contains the square of the actual f-number we saved for later.
frndint rounds the TOS to the nearest integer. fld1
pushes a 1. fscale shifts the
1 we have on the TOS by
the value in st(1), effectively raising 2 to st(1).
Finally, fsqrt calculates the square root of the result,
i.e., the nearest normalized f-number.
We now have the nearest normalized f-number on the TOS, the base-2 logarithm rounded to the nearest integer in st(1), and the square of the actual f-number in st(2). We are saving the value in st(2) for later.
But we do not need the contents of st(1) anymore. The last
line, fstp st1, places the contents of st to st(1), and pops. As a result, what
was st(1) is now st, what was st(2) is now st(1), etc. The new st contains the normalized f-number. The new st(1) contains the square of the actual f-number we have stored
there for posterity.
At this point, we are ready to output the normalized f-number. Because it is normalized, we will not round it off to four significant digits, but will send it out in its full precision.
The normalized f-number is useful as long as it is reasonably small and can be found on our light meter. Otherwise we need a different method of determining proper exposure.
Earlier we have figured out the formula of calculating proper exposure at an arbitrary f-number from that measured at a different f-number.
Every light meter I have ever seen can determine proper exposure at f5.6. We will, therefore, calculate an "f5.6 multiplier," i.e., by how much we need to multiply the exposure measured at f5.6 to determine the proper exposure for our pinhole camera.
From the above formula we know this factor can be calculated by dividing our f-number
(the actual one, not the normalized one) by 5.6, and
squaring the result.
Mathematically, dividing the square of our f-number by the square of 5.6 will give us the same result.
Computationally, we do not want to square two numbers when we can only square one. So, the first solution seems better at first.
But...
5.6 is a constant. We do not have to have our FPU waste precious cycles. We can just tell it to divide the
square of the f-number by whatever 5.6² equals to. Or we
can divide the f-number by 5.6, and then square the result.
The two ways now seem equal.
But, they are not!
Having studied the principles of photography above, we remember that the 5.6 is actually square root of 2 raised to the fifth power. An
irrational number. The square of
this number is exactly 32.
Not only is 32 an integer, it is a power of 2. We do not
need to divide the square of the f-number by 32. We only
need to use fscale to shift it right by five positions. In
the FPU lingo it means we will fscale it with st(1) equal to -5. That is much
faster than a division.
So, now it has become clear why we have saved the square of the f-number on the top of the FPU stack. The calculation of the f5.6 multiplier is the easiest calculation of this entire program! We will output it rounded to four significant digits.
There is one more useful number we can calculate: The number of stops our f-number is from f5.6. This may help us if our f-number is just outside the range of our light meter, but we have a shutter which lets us set various speeds, and this shutter uses stops.
Say, our f-number is 5 stops from f5.6, and the light meter says we should use 1/1000 sec. Then we can set our shutter speed to 1/1000 first, then move the dial by 5 stops.
This calculation is quite easy as well. All we have to do is to calculate the base-2 logarithm of the f5.6 multiplier we had just calculated (though we need its value from before we rounded it off). We then output the result rounded to the nearest integer. We do not need to worry about having more than four significant digits in this one: The result is most likely to have only one or two digits anyway.
11.13.4 FPU Optimizations
In assembly language we can optimize the FPU code in ways impossible in high languages, including C.
Whenever a C function needs to calculate a floating-point value, it loads all necessary variables and constants into FPU registers. It then does whatever calculation is required to get the correct result. Good C compilers can optimize that part of the code really well.
It "returns" the value by leaving the result on the TOS. However, before it returns, it cleans up. Any variables and constants it used in its calculation are now gone from the FPU.
It cannot do what we just did above: We calculated the square of the f-number and kept it on the stack for later use by another function.
We knew we would need that value later on. We also knew we had enough room on the stack (which only has room for 8 numbers) to store it there.
A C compiler has no way of knowing that a value it has on the stack will be required again in the very near future.
Of course, the C programmer may know it. But the only recourse he has is to store the value in a memory variable.
That means, for one, the value will be changed from the 80-bit precision used internally by the FPU to a C double (64 bits) or even single (32 bits).
That also means that the value must be moved from the TOS into the memory, and then back again. Alas, of all FPU operations, the ones that access the computer memory are the slowest.
So, whenever programming the FPU in assembly language, look for the ways of keeping intermediate results on the FPU stack.
We can take that idea even further! In our program we are using a constant (the one we named PC).
It does not matter how many pinhole diameters we are calculating: 1, 10, 20, 1000, we are always using the same constant. Therefore, we can optimize our program by keeping the constant on the stack all the time.
Early on in our program, we are calculating the value of the above constant. We need
to divide our input by 10 for every digit in the
constant.
It is much faster to multiply than to divide. So, at the start of our program, we
divide 10 into 1 to obtain
0.1, which we then keep on the stack: Instead of dividing
the input by 10 for every digit, we multiply it by 0.1.
By the way, we do not input 0.1 directly, even though we
could. We have a reason for that: While 0.1 can be
expressed with just one decimal place, we do not know how many binary places it takes. We, therefore, let the FPU calculate its binary value to its own high precision.
We are using other constants: We multiply the pinhole diameter by 1000 to convert it from millimeters to microns. We compare
numbers to 10000 when we are rounding them off to four
significant digits. So, we keep both, 1000 and 10000, on the stack. And, of course, we reuse the 0.1 when rounding off numbers to four digits.
Last but not least, we keep -5 on the stack. We need it
to scale the square of the f-number, instead of dividing it by 32. It is not by coincidence we load this constant last. That
makes it the top of the stack when only the constants are on it. So, when the square of
the f-number is being scaled, the -5 is at st(1), precisely where fscale expects
it to be.
It is common to create certain constants from scratch instead of loading them from the
memory. That is what we are doing with -5:
fld1 ; TOS = 1
fadd st0, st0 ; TOS = 2
fadd st0, st0 ; TOS = 4
fld1 ; TOS = 1
faddp st1, st0 ; TOS = 5
fchs ; TOS = -5
We can generalize all these optimizations into one rule: Keep repeat values on the stack!
Tip: PostScript® is a stack-oriented programming language. There are many more books available about PostScript than about the FPU assembly language: Mastering PostScript will help you master the FPU.
11.13.5 pinhole--The Code
;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Find various parameters of a pinhole camera construction and use
;
; Started: 9-Jun-2001
; Updated: 10-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
align 4
ten dd 10
thousand dd 1000
tthou dd 10000
fd.in dd stdin
fd.out dd stdout
envar db 'PINHOLE=' ; Exactly 8 bytes, or 2 dwords long
pinhole db '04,', ; Bender's constant (0.04)
connors db '037', 0Ah ; Connors' constant
usg db 'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "pinhole: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "pinhole: Can't create output file", 0Ah
oemlen equ $-oemsg
pinmsg db "pinhole: The PINHOLE constant must not be 0", 0Ah
pinlen equ $-pinmsg
toobig db "pinhole: The PINHOLE constant may not exceed 18 decimal places", 0Ah
biglen equ $-toobig
huhmsg db 9, '???'
separ db 9, '???'
sep2 db 9, '???'
sep3 db 9, '???'
sep4 db 9, '???', 0Ah
huhlen equ $-huhmsg
header db 'focal length in millimeters,pinhole diameter in microns,'
db 'F-number,normalized F-number,F-5.6 multiplier,stops '
db 'from F-5.6', 0Ah
headlen equ $-header
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
dbuffer resb 20 ; decimal input buffer
bbuffer resb 10 ; BCD buffer
section .text
align 4
huh:
call write
push dword huhlen
push dword huhmsg
push dword [fd.out]
sys.write
add esp, byte 12
ret
align 4
perr:
push dword pinlen
push dword pinmsg
push dword stderr
sys.write
push dword 4 ; return failure
sys.exit
align 4
consttoobig:
push dword biglen
push dword toobig
push dword stderr
sys.write
push dword 5 ; return failure
sys.exit
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
sub esi, esi
.arg:
pop ecx
or ecx, ecx
je near .getenv ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
inc ecx
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .c
or ah, ah
jne .pcheck
pop ecx
or ecx, ecx
je near usage
mov ah, [ecx]
.pcheck:
cmp ah, '0'
jl near usage
cmp ah, '9'
ja near usage
mov esi, ecx
jmp .arg
.c:
cmp al, 'c'
jne .b
or ah, ah
jne near usage
mov esi, connors
jmp .arg
.b:
cmp al, 'b'
jne .e
or ah, ah
jne near usage
mov esi, pinhole
jmp .arg
.e:
cmp al, 'e'
jne near usage
or ah, ah
jne near usage
mov al, ','
mov [huhmsg], al
mov [separ], al
mov [sep2], al
mov [sep3], al
mov [sep4], al
jmp .arg
align 4
.getenv:
; If ESI = 0, we did not have a -p argument,
; and need to check the environment for "PINHOLE="
or esi, esi
jne .init
sub ecx, ecx
.nextenv:
pop esi
or esi, esi
je .default ; no PINHOLE envar found
; check if this envar starts with 'PINHOLE='
mov edi, envar
mov cl, 2 ; 'PINHOLE=' is 2 dwords long
rep cmpsd
jne .nextenv
; Check if it is followed by a digit
mov al, [esi]
cmp al, '0'
jl .default
cmp al, '9'
jbe .init
; fall through
align 4
.default:
; We got here because we had no -p argument,
; and did not find the PINHOLE envar.
mov esi, pinhole
; fall through
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
sub edx, edx
mov edi, dbuffer+1
mov byte [dbuffer], '0'
; Convert the pinhole constant to real
.constloop:
lodsb
cmp al, '9'
ja .setconst
cmp al, '0'
je .processconst
jb .setconst
inc dl
.processconst:
inc cl
cmp cl, 18
ja near consttoobig
stosb
jmp short .constloop
align 4
.setconst:
or dl, dl
je near perr
finit
fild dword [tthou]
fld1
fild dword [ten]
fdivp st1, st0
fild dword [thousand]
mov edi, obuffer
mov ebp, ecx
call bcdload
.constdiv:
fmul st0, st2
loop .constdiv
fld1
fadd st0, st0
fadd st0, st0
fld1
faddp st1, st0
fchs
; If we are creating a CSV file,
; print header
cmp byte [separ], ','
jne .bigloop
push dword headlen
push dword header
push dword [fd.out]
sys.write
.bigloop:
call getchar
jc near done
; Skip to the end of the line if you got '#'
cmp al, '#'
jne .num
call skiptoeol
jmp short .bigloop
.num:
; See if you got a number
cmp al, '0'
jl .bigloop
cmp al, '9'
ja .bigloop
; Yes, we have a number
sub ebp, ebp
sub edx, edx
.number:
cmp al, '0'
je .number0
mov dl, 1
.number0:
or dl, dl ; Skip leading 0's
je .nextnumber
push eax
call putchar
pop eax
inc ebp
cmp ebp, 19
jae .nextnumber
mov [dbuffer+ebp], al
.nextnumber:
call getchar
jc .work
cmp al, '#'
je .ungetc
cmp al, '0'
jl .work
cmp al, '9'
ja .work
jmp short .number
.ungetc:
dec esi
inc ebx
.work:
; Now, do all the work
or dl, dl
je near .work0
cmp ebp, 19
jae near .toobig
call bcdload
; Calculate pinhole diameter
fld st0 ; save it
fsqrt
fmul st0, st3
fld st0
fmul st5
sub ebp, ebp
; Round off to 4 significant digits
.diameter:
fcom st0, st7
fstsw ax
sahf
jb .printdiameter
fmul st0, st6
inc ebp
jmp short .diameter
.printdiameter:
call printnumber ; pinhole diameter
; Calculate F-number
fdivp st1, st0
fld st0
sub ebp, ebp
.fnumber:
fcom st0, st6
fstsw ax
sahf
jb .printfnumber
fmul st0, st5
inc ebp
jmp short .fnumber
.printfnumber:
call printnumber ; F number
; Calculate normalized F-number
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
sub ebp, ebp
call printnumber
; Calculate time multiplier from F-5.6
fscale
fld st0
; Round off to 4 significant digits
.fmul:
fcom st0, st6
fstsw ax
sahf
jb .printfmul
inc ebp
fmul st0, st5
jmp short .fmul
.printfmul:
call printnumber ; F multiplier
; Calculate F-stops from 5.6
fld1
fxch st1
fyl2x
sub ebp, ebp
call printnumber
mov al, 0Ah
call putchar
jmp .bigloop
.work0:
mov al, '0'
call putchar
align 4
.toobig:
call huh
jmp .bigloop
align 4
done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
finit
; return success
push dword 0
sys.exit
align 4
skiptoeol:
; Keep reading until you come to cr, lf, or eof
call getchar
jc done
cmp al, 0Ah
jne .cr
ret
.cr:
cmp al, 0Dh
jne skiptoeol
ret
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
clc
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .empty
sub eax, eax
ret
align 4
.empty:
add esp, byte 4
stc
ret
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
align 4
bcdload:
; EBP contains the number of chars in dbuffer
push ecx
push esi
push edi
lea ecx, [ebp+1]
lea esi, [dbuffer+ebp-1]
shr ecx, 1
std
mov edi, bbuffer
sub eax, eax
mov [edi], eax
mov [edi+4], eax
mov [edi+2], ax
.loop:
lodsw
sub ax, 3030h
shl al, 4
or al, ah
mov [edi], al
inc edi
loop .loop
fbld [bbuffer]
cld
pop edi
pop esi
pop ecx
sub eax, eax
ret
align 4
printnumber:
push ebp
mov al, [separ]
call putchar
; Print the integer at the TOS
mov ebp, bbuffer+9
fbstp [bbuffer]
; Check the sign
mov al, [ebp]
dec ebp
or al, al
jns .leading
; We got a negative number (should never happen)
mov al, '-'
call putchar
.leading:
; Skip leading zeros
mov al, [ebp]
dec ebp
or al, al
jne .first
cmp ebp, bbuffer
jae .leading
; We are here because the result was 0.
; Print '0' and return
mov al, '0'
jmp putchar
.first:
; We have found the first non-zero.
; But it is still packed
test al, 0F0h
jz .second
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
.second:
add al, '0'
call putchar
.next:
cmp ebp, bbuffer
jb .done
mov al, [ebp]
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
add al, '0'
call putchar
dec ebp
jmp short .next
.done:
pop ebp
or ebp, ebp
je .ret
.zeros:
mov al, '0'
call putchar
dec ebp
jne .zeros
.ret:
ret
The code follows the same format as all the other filters we have seen before, with one subtle exception:
We are no longer assuming that the end of input implies the end of things to do, something we took for granted in the character-oriented filters.
This filter does not process characters. It processes a language (albeit a very simple one, consisting only of numbers).
When we have no more input, it can mean one of two things:
We are done and can quit. This is the same as before.
The last character we have read was a digit. We have stored it at the end of our ASCII-to-float conversion buffer. We now need to convert the contents of that buffer into a number and write the last line of our output.
For that reason, we have modified our
getcharand ourreadroutines to return with the carry flag clear whenever we are fetching another character from the input, or the carry flag set whenever there is no more input.Of course, we are still using assembly language magic to do that! Take a good look at
getchar. It always returns with the carry flag clear.Yet, our main code relies on the carry flag to tell it when to quit--and it works.
The magic is in
read. Whenever it receives more input from the system, it just returns togetchar, which fetches a character from the input buffer, clears the carry flag and returns.But when
readreceives no more input from the system, it does not return togetcharat all. Instead, theadd esp, byte 4op code adds4to ESP, sets the carry flag, and returns.So, where does it return to? Whenever a program uses the
callop code, the microprocessorpushes the return address, i.e., it stores it on the top of the stack (not the FPU stack, the system stack, which is in the memory). When a program uses theretop code, the microprocessorpops the return value from the stack, and jumps to the address that was stored there.But since we added
4to ESP (which is the stack pointer register), we have effectively given the microprocessor a minor case of amnesia: It no longer remembers it wasgetcharthatcalledread.And since
getcharneverpushed anything beforecallingread, the top of the stack now contains the return address to whatever or whoevercalledgetchar. As far as that caller is concerned, hecalledgetchar, whichreturned with the carry flag set!
Other than that, the bcdload routine is caught up in the
middle of a Lilliputian conflict between the Big-Endians and the Little-Endians.
It is converting the text representation of a number into that number: The text is stored in the big-endian order, but the packed decimal is little-endian.
To solve the conflict, we use the std op code early on.
We cancel it with cld later on: It is quite important we do
not call anything that may depend on the default setting of
the direction flag while std is active.
Everything else in this code should be quite clear, providing you have read the entire chapter that precedes it.
It is a classical example of the adage that programming requires a lot of thought and only a little coding. Once we have thought through every tiny detail, the code almost writes itself.
11.13.6 Using pinhole
Because we have decided to make the program ignore any input except for numbers (and even those inside a comment), we can actually perform textual queries. We do not have to, but we can.
In my humble opinion, forming a textual query, instead of having to follow a very strict syntax, makes software much more user friendly.
Suppose we want to build a pinhole camera to use the 4x5 inch film. The standard focal length for that film is about 150mm. We want to fine-tune our focal length so the pinhole diameter is as round a number as possible. Let us also suppose we are quite comfortable with cameras but somewhat intimidated by computers. Rather than just have to type in a bunch of numbers, we want to ask a couple of questions.
Our session might look like this:
% pinhole Computer, What size pinhole do I need for the focal length of 150? 150 490 306 362 2930 12 Hmmm... How about 160? 160 506 316 362 3125 12 Let's make it 155, please. 155 498 311 362 3027 12 Ah, let's try 157... 157 501 313 362 3066 12 156? 156 500 312 362 3047 12 That's it! Perfect! Thank you very much! ^D
We have found that while for the focal length of 150, our pinhole diameter should be 490 microns, or 0.49 mm, if we go with the almost identical focal length of 156 mm, we can get away with a pinhole diameter of exactly one half of a millimeter.
11.13.7 Scripting
Because we have chosen the # character to denote the
start of a comment, we can treat our pinhole software as a
scripting language.
You have probably seen shell scripts that start with:
#! /bin/sh
...or...
#!/bin/sh
...because the blank space after the #! is optional.
Whenever UNIX is asked to run an executable file which
starts with the #!, it assumes the file is a script. It
adds the command to the rest of the first line of the script, and tries to execute
that.
Suppose now that we have installed pinhole in /usr/local/bin/, we can now write a script to calculate various pinhole diameters suitable for various focal lengths commonly used with the 120 film.
The script might look something like this:
#! /usr/local/bin/pinhole -b -i # Find the best pinhole diameter # for the 120 film ### Standard 80 ### Wide angle 30, 40, 50, 60, 70 ### Telephoto 100, 120, 140
Because 120 is a medium size film, we may name this file medium.
We can set its permissions to execute, and run it as if it were a program:
% chmod 755 medium % ./medium
UNIX will interpret that last command as:
% /usr/local/bin/pinhole -b -i ./medium
It will run that command and display:
80 358 224 256 1562 11 30 219 137 128 586 9 40 253 158 181 781 10 50 283 177 181 977 10 60 310 194 181 1172 10 70 335 209 181 1367 10 100 400 250 256 1953 11 120 438 274 256 2344 11 140 473 296 256 2734 11
Now, let us enter:
% ./medium -c
UNIX will treat that as:
% /usr/local/bin/pinhole -b -i ./medium -c
That gives it two conflicting options: -b and -c (Use Bender's constant and use Connors' constant). We have programmed it so later options override early ones--our program will calculate everything using Connors' constant:
80 331 242 256 1826 11 30 203 148 128 685 9 40 234 171 181 913 10 50 262 191 181 1141 10 60 287 209 181 1370 10 70 310 226 256 1598 11 100 370 270 256 2283 11 120 405 296 256 2739 11 140 438 320 362 3196 12
We decide we want to go with Bender's constant after all. We want to save its values as a comma-separated file:
% ./medium -b -e > bender % cat bender focal length in millimeters,pinhole diameter in microns,F-number,normalized F-number,F-5.6 multiplier,stops from F-5.6 80,358,224,256,1562,11 30,219,137,128,586,9 40,253,158,181,781,10 50,283,177,181,977,10 60,310,194,181,1172,10 70,335,209,181,1367,10 100,400,250,256,1953,11 120,438,274,256,2344,11 140,473,296,256,2734,11 %
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Generated: 2007-01-26 17:58:41