Difference between revisions of "Floating-point Opcodes"
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Latest revision as of 05:51, 9 July 2020
The FPU offers a lot of complex and powerful floating-point operations, like
SQRT, etc. (SIMPLY FPU by Raymond Filiatreault has a compact overview of all FPU commands for the curious.) Use of the FPU can drastically increase what you can do in a tiny production while simultaneously keeping code size down. All x86 CPUs can use the FPU, although older CPUs (80486sx and earlier) need the FPU physically installed in the FPU socket.
This guide assumes intermediate to advanced-level proficiency with sizecoding.
- 1 FPU Basics
- 2 Optimizing with the FPU
- 3 FPU Tricks
Usage and communication with the FPU is quite uncommon and takes a bit to get used to, so we'll start with a simple example. This is what our code example looks like originally:
cwd ; "clear" DX for perfect alignment mov al,0x13 X: int 0x10 ; set video mode AND draw pixel mov ax,cx ; get column in AH add ax,di ; offset by framecounter <-- REPLACE THIS WITH FPU CODE xor al,ah ; the famous XOR pattern and al,32+8 ; a more interesting variation of it mov ah,0x0C ; set subfunction "set pixel" for int 0x10 loop X ; loop 65536 times inc di ; increment framecounter in al,0x60 ; check keyboard ... dec al ; ... for ESC jnz X ; rinse and repeat ret ; quit program
and this is how the code looks if we replace the instruction with FPU code :
cwd ; "clear" DX for perfect alignment mov al,0x13 X: int 0x10 ; set video mode AND draw pixel mov ax,cx ; get "column" in AX fninit ; init FPU first mov [si],ax ; write first addend to a memory location fild word [si] ; F(pu) I(nteger) L(oad)D a WORD from memory location to the FPU stack mov [si],di ; write second addend to a memory location fiadd word [si] ; Directly add the word in the memory location to the top FPU stack fist word [si] ; F(pu) I(nteger) ST(ore) the result into a memory location mov ax,[si] ; Get the word from the memory location into AX xor al,ah ; the famous XOR pattern and al,32+8 ; a more interesting variation of it mov ah,0x0C ; set subfunction "set pixel" for int 0x10 loop X ; loop 65536 times inc di ; increment framecounter in al,0x60 ; check keyboard ... dec al ; ... for ESC jnz X ; rinse and repeat ret ; quit program
(Obviously that made the program bigger instead of smaller, but the point of this exercise is to illustrate how to work with the FPU.) Looking at our changes, you get a sense of what usual interaction with the FPU is:
F(N)INIT: Initialize the FPU
- transfer values from CPU registers to memory location(s)
- transfer values from memory location(s) onto FPU stack
- do the actual calculations using the FPU (more on this soon)
- transfer result from the FPU stack into memory location(s)
- transfer result from memory location(s) back into registers
That is a lot of extra code for a single integer addition, but that's a simple example; once more complex floating point operations are involved, it starts to pay off.
Optimizing with the FPU
Distance function example
For more advanced FPU operation, let's start from scratch with an unoptimized program which plots the distance of each pixel to the screen center as color, in 49 bytes.
push 0a000h pop es ; get start of video memory in ES mov al,0x13 ; switch to video mode 13h int 0x10 ; 320 * 200 in 256 colors fninit ; - ; it's useful to comment what's on the ; stack after each FPU operation ; to not get lost ;) start is : empty (-) X: xor dx,dx ; reset the high word before division mov bx,320 ; 320 columns mov ax,di ; get screen pointer in AX div bx ; construct X,Y from screen pointer into AX,DX sub ax,100 ; subtract the origin sub dx,160 ; = (160,100) ... center of 320x200 screen mov [si],ax ; move X into a memory location fild word [si] ; X fmul st0 ; X² mov [si],dx ; move Y into a memory location fild word [si] ; Y X² fmul st0 ; Y² X² fadd st0,st1 ; Y²+X² fsqrt ; R fistp word [si] ; - mov ax,[si] ; get the result from memory stosb ; write to screen (DI) and increment DI jmp short X ; next pixel
A few words on this :
- The FPU registers (st0, st1, ...) are organized as a stack. When you load something to the FPU, everything else will be moved one location further away from the top (implicitly!) Some FPU instructions work only on the top, other allow the explicit parametrization with arbitrary FPU registers.
- Depending on what you do, sometimes
F(N)INITcan be omitted. Real hardware will refuse to work more often than emulators, but it's always worth the try.
- Accessing memory (size) efficiently can be a real pain. The safest way is to reference absolute memory locations (f.e
) but that's two bytes more per instruction than referencing memory with
[BX+DI]. When working with FPU and this classic approach of FPU communication, you have to design your codeflow to have one or some of these locations available.
- Accessing the memory is always with regard to the segment register
DSunless you perform segment overrides. When accessing memory with
[BP+??]be aware that the memory is accessed with regard to the segment register
SS(see Register Indirect Addressing Modes)
- There are a few conventions which help you identify FPU commands. "i" stands for integer (WORD or DWORD), "p" means "pop stack afterwards", so
FSTmeans just "store" while
FISTPmeans "store as integer, then pop the stack"
Stack addressing, "Rrrolas Trick", alignment optimization, Aspect Ratio
Now let's unleash the state of the art sizecoding arsenal onto this, to bring it down to 37 bytes (40 bytes with aspect correction)
push 0a000h - 70 ; modified to center to 160,100 aas ; aspect ratio constant part pop es ; get start of video memory in ES mov al,0x13 ; switch to video mode 13h int 0x10 ; 320 * 200 in 256 colors X: mov ax,0xCCCD ; perform the famous... mul di ; ... Rrrola trick =) sub dh,[si] ; align vertically pusha ; push all registers on stack fild word [bx-8] ; X fmul st0 ; X² fild word [bx-9] ; Y X² fmul dword [bx+si] ; aspect ratio correction fmul st0 ; Y² X² fadd st0,st1 ; Y²+X² fsqrt ; R fistp dword [bx-5] ; - popa ; pop all registers from stack stosb ; write to screen (DI) and increment DI jmp short X ; next pixel
The resulting image is almost identical to to the former. Let's go through this step by step:
push 0a000h - 70
Instead of aligning horizontally with
sub dx,160 we can code this implicitly by moving our segment register ten units - that is 10 * 16 = 160 pixels - to the left (see Real Mode Addressing). With further multiple subtraction of 20 units - that is 320 pixels, we can shift the visible screen towards the top, to finetune vertical alignment. As long as this shift is no more than 4 lines ( 65536 / 320 - 200 = 4,8 ) there is no further visual impact.
This is the high byte of a constant, placed in a way that
[BX+SI] resolves to ~1.24 when read as 32bit float. The last byte of segment
ES is also of importance. Check yourself with the IEEE 754 Converter
mul di(The "Rrrola trick")
Instead of constructing X and Y from the screen pointer
DIV you can get a decent estimation with multiplying the screen pointer with
0xCCCD and read X and Y from the 8bit registers
DH (+DL as 16bit value) and
DL (+AH as 16bit value). The idea is to interpret
DI as a kind of 16 bit float in the range
[0,1], from start to end. Multiplying this number in [0,1] with 65536 / 320 = 204,8 results in the row before the comma, and again as a kind of a float, the column after the comma. The representation
0xCCCD is the nearest rounding of 204,8 * 256 ( = 52428,8 ~ 52429 = 0xCCCD). As long as the 16 bit representations are used, there is no precision loss.
The instruction at
push <word> and has the opcode
0x68 which is 104 in decimal. Combined with the fine tuned vertical alignment above ( ~4 lines) this results in (virtually) subtracting 100 for perfect vertical alignment. This is one byte shorter than
pusha / popa
Instead of going the classical way of communicating with the FPU, we push all the registers, read/write values with memory addressing to/from the FPU, then pop all registers again. This works when
SP is "close enough" to
BX (initially zero and kept that way) to allow
[BX+<signed byte>] addressing. It comes with the special benefit of implicit 8bit shifts. One serious drawback is loss of precision, since the registers
AH "lose connection" when using
PUSHA (see the order of registers : PUSHA/PUSHAD documentation
fild word [bx+<signed byte>]& *
fistp dword [bx+<signed byte>]
This is the so called "stack addressing". We assume that
SP=0xFFFE at start, so we know where the registers are in memory after
pusha (AX at [BX-4], CX at [BX-6] etc.). It's important to realize that we work with signed 16 bit values now, in the full range of [-32768,32767]. That is also why we need
DWORD when storing the result :
sqrt(x²+y²) exceeds the signed 16bit range for quite some value pairs. Note that there are already implicit 8bit shifts (bx-9,bx-5)
fmul dword [bx+si]
With the "Rrrola" trick above, we have the row number to be 204 at maximum, but also the column can't be greater than 256. This results in a wrong aspect ratio, but it can almost completely be fixed with this two byte instruction (+ one byte for the
AAS instruction) : 256 * 1,24 = 317,44 which is quite close to 320. If aspect ratio is of no meaning to the desired effect, this three bytes can be shaved off.
Now let's add some features:
- extract angle as opposed to the distance and combine both
- reverse divide the distance to create the "tunnel" effect
- animate with smooth steps along the distance
- improve on the colors with subselecting from the standard palette
- quit the program on ESC
This results in the following program with a size of 63 bytes :
push 0xa000 - 10 - 3 * 20 ; video base - 3.5 lines or al, 0x13 ; mode 13h = 320 x 200 in 256 colors pop es ; get aligned video memory base int 0x10 ; switch videomode X: sub dh, [si] ; vertical alignment pusha ; push all registers on stack fild word [bx-9] ; fpustack : x fild word [bx-8] ; fpustack : y x fpatan ; fpustack : arc fst st1 ; fpustack : arc arc fcos ; fpustack : cos(arc) arc fimul dword [si] ; fpustack : l*cos(arc) arc fidiv word [bx-8] ; fpustack : l*cos(arc)/x arc fiadd word [bp+si] ; fpustack : l*cos(arc)/x+offset arc fistp dword [bx-7] ; fpustack : arc fimul word [byte si+val] ; fpustack : scaled_arc fistp word [bx-5] ; fpustack : - popa ; pop all registers from stack xor al, cl ; XOR scaled_arc with distance and al, 16 + 8 + 2 ; sub selecting palette part stosb ; writing to screen mov ax, 0xCCCD ; Performing the famous mul di ; Rrrola trick jo X ; next frame check add word [bp+si], byte 23 ; change offset smoothly in al, 0x60 ; check for ... dec ax ; ...ESC key jnz X ; otherwise continue ret ; quit program val: dw 6519 ; n = 160 * 256 / pi / 2 ; 0x1977
Many other tiny tunnel effects have been coded, so it is highly recommended to check out the documented source code of "Constant Evolution" by ryg/Farbrausch and the "Heart shaped tunnel" from Lord Kelvin, both with a size of 64 bytes. While "Constant Evolution" takes a slightly different route than the example here (classic FPU communication, classic X Y construction, sqrt(x²+y²) instead of using
fimul), "Heart shaped tunnel" uses no FPU at all.
The takeaways from this example are:
- Loading a constant from the code with some degrees of freedom
fimul dword [si] multiplies with a 32 bit integer dividend for the tunnel effect. The highest byte of this constant points to our code, to the opcode from
or al, 0x13. This instruction puts
al, and since there a lot of possibilities to achieve this, there is a direct way of changing the appearance of the tunnel with changing this instruction to one of the following :
sbb al,0xED or
sub al,0xED. In this special case, the instruction can also be swapped with
pop es to gain a further degree of freedom.
- normal loading of a constant which can't be reused as opcode
Although it's the ultimate goal to not even use a single extra byte for constants, sometimes the required sequence simply does not appear in the code. In this case, a constant is needed to convert the angle from the range [-pi,pi] to the color space in a way that no gaps appear while stepping from 359° to 0°. In the last line at
val: the value
160 in the comment is 32 * 5 where 5 is number of "spiral arms" the tunnel effect has. The 8bit shift (*256) is to increase precision. It turns out that 16bit precision is enough to get a decent "gap closer" for values obtained by
- operating directly on an indirect memory location without offset
[bp+si] is used as animation variable, while both participatory registers are kept fixed (the value is
0x0A?? and therefor way above our code). Since we work with 16bit values and the top 8bit are the measurement in pixels, the instruction
add word [bp+si], byte 23 allows for sub pixel precision in animation, while occupying 3 bytes of space. Depending on the target hardware, this value 23 can be increased/decreased to achieve faster/smoother animation.
- Optimizing the check for the next frame
Normally, there is a check like
test di,di with direct consecutive branch necessary. The used approach allows for direct branching after
mul di with
jo, since the overflow flag is always but twice triggered for a frame. This saves two bytes, but also requires adjustment of the animation constant, because the animation constant is also added twice. A further benefit is that in one of these two cases,
AX is zero which save a further byte on the following ESC check (
dec ax instead of
dec al )
Size optimizing the "Tunnel"
Now if we abandon all the comfort, alignment, smoothness and convenience, and optimize this straight for size, we end up with a 52 byte version. This does not include the possible exclusion of color tuning (2 bytes), after all the effect is supposed to look at least somewhat appealing ;)
mov al,0x13 ; mode 13h = 320 x 200 in 256 colors int 0x10 ; switch videomode X: or al, [bp+si] ; *illusion* - executed ONCE xor al, 0x68 ; *illusion* - executed ONCE mov dx, 0x79F ; *illusion* - executed ONCE pusha ; push all registers on stack fild word [bx-9] ; x fild word [bx-8] ; y x fpatan ; arc fst st1 ; arc arc fcos ; cos(arc) arc fimul dword [si] ; l*cos(arc) arc fidiv word [bx-8] ; l*cos(arc)/x arc fistp dword [bx-4] ; arc fimul word [bx] ; scaled_arc fistp word [bx-5] ; - popa ; pop all registers from stack sub ah, [bp+si] ; animation along distance xor al, ah ; XOR scaled_arc with distance and al, 16 + 8 + 4 ; sub palette selection stosb ; write to screen, advance DI mov ax, 0xCCCD ; the famous mul di ; Rrrola trick jmp short X-1 ; *ODD* jump into "int 0x10"
Since this code contains an "odd jump" into the middle of the instruction
int 0x10 it helps to disassemble the code from address
0x103 on, until both code pathes realign at
adc [bp+si],cl ; decrement framecounter add dh,[si] ; vertical alignment push word 0x9FBA ; video base - 3.5 lines pop es ; get aligned video memory ; continues with "pusha"
Like before the
mul di instruction triggers the overflow flag - and the carry flag - always but twice per frame.
0xFF unchanged from start, so
adc [bp+si],cl effectively decrements the framecounter twice per frame.
The dividend for scaling the arc is now taken from the location
[bx], which is
0x20CD. Calculating the number of "spirals" backwards from this number (*pi*2/256/32) gives about 6.44 which is close enough to 6.5, so that the visual gap in the top is almost not recognizable - besides from the miscoloring which can be fixed by removing the 16 from
and al, 16 + 8 + 4.
It's noteworthy that the location
[si] does not contain a good offset anymore, since
push <word> moved away from the top, to help create *illusion* code that not only realigns soon in the second code path but also does not modify registers and memory environment in a hindering way (only
DX are modified)
The takeaway here is to study the modbytes of instructions that are very likely to appear in tiny intros, as well as using their constants as code.
int 0x10 will almost always be in your code, so there is always
adc byte[??], 8bit_reg too in between.
Finally, you might have noticed that these tunnels don't "spin". For the cost of two bytes this can be easily added.
add al, [bp+si] before applying XOR in the last example would offset the
arc with the framecounter and produce a spinning effect. For a coherent visual experience the sub palette selection must be changed to
and al, 8 + 4, too.
Comparing 2 float numbers on the FPU
Sometimes when doing fpu algebra, you need to compare 2 float numbers using the FPU (for example when calculating an intersection between a point and an object).
To do so will take a few steps:
fcomp ; compare the contents of ST0 and ST1 fstsw ax ; copy the fpu flags to ax (ah) sahf ; copy the contens of AH register to the CPU-flags jb inside ; you can now check the cpu flags and jump accordingly
If a Pentium Pro instruction set is available or emulated (Real DOS setups or Dosbox-X), you can also use the FCOMI instruction instead to do the same thing, which saves a few bytes:
fcomip jb inside
Truncate float numbers / get fractional part
May be you would want to have the truncated number of your floating point value for some purpose or you would want the fractional part. In that case an instruction comes in handy that was added quite late to the FPU. It came with the SSE3 insutrction set and is called
fisttp (Store Integer with Truncation).
To get the fractional part you can do it like this:
fild st0 ;duplicate your number => st0=f, st1=f fisttp dword[si] ;INT(f) - store truncated value somewhere in memory fild dword[si] ;load truncated value fsubp st1,st0 ;calculate f-INT(f)
Generally it's shorter and faster than fiddling arount with rounding mode and
Simple Floor Casting
Another classic effect that works well with the FPU is a simple Perspective Floor Caster. While this effect can also be achieved with just CPU code, using the FPU gives a bit finer control over things like camera height and other things.
Here is the basic code for the effect as used in e.g. Rush, by: Marquee Design.
fninit fild word [bx-8] ; load y-value fidiv word [viewheight] ; scalingvalue/height to plane fabs ; abs(y) fild word [bx-9] ; x abs(y) fdiv st1 ; x/abs(y) abs(y) fistp word [bx-4] ; store U (plane x) to ax fidivr word [floorval] ; 32767/abs(y) fist word [bx-6] ; store V (plane y) to cx
The following constants are used in the above calculation:
floorval dw 32767 viewheight dw 10
Needless to say, the code above is simplified and unoptimised or educational purposes. It can be optimised further using various tricks mentioned above and integrated easily into already existing FPU calculations for example to save space.