Difference between revisions of "Floating-point Opcodes"
From SizeCoding
(Created page with " The FPU offers a lot of operations not available to classic x86 CPU, like <code>SIN</code>, <code>COS</code>, <code>TAN</code>, <code>EXP</code> and so on. Usage and communic...") |
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| − | The FPU offers a lot of operations not available to classic x86 CPU, like <code>SIN</code>, <code>COS</code>, <code>TAN</code>, <code>EXP</code> and so on. Usage and communication with the FPU is a bit | + | The FPU offers a lot of operations not available to classic x86 CPU, like <code>SIN</code>, <code>COS</code>, <code>TAN</code>, <code>EXP</code>, <code>SQRT</code>, <code>LN</code> and so on. Usage and communication with the FPU is a bit uncommon and takes a bit to get used to. It's recommended to read the creation of the [[Output#Outputting_in_mode_13h_.28320x200.29|snippet we want to modify]] first, this is how it looks like originally : |
<syntaxhighlight lang="nasm">cwd ; "clear" DX for perfect alignment | <syntaxhighlight lang="nasm">cwd ; "clear" DX for perfect alignment | ||
| Line 42: | Line 42: | ||
ret ; quit program</syntaxhighlight> | ret ; quit program</syntaxhighlight> | ||
| − | ( | + | The usual interaction with the FPU is as follows |
| + | * <code>F(N)INIT</code> : Initialization of the FPU | ||
| + | * store register content in memory location(s) | ||
| + | * transfer from memory location onto FPU stack | ||
| + | * actual calculations on the FPU (more on this soon) | ||
| + | * transfer from FPU stack into memory location(s) | ||
| + | * get register from memory location | ||
| + | |||
| + | That would be a lot for a single integer addition, but once more complex floating point operations are involved, it starts to pay off. 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. | ||
| + | |||
| + | [[File:Distance to center example.png|thumb]] | ||
| + | |||
| + | <syntaxhighlight lang="nasm">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 screenpointer 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] ; | ||
| + | stosb ; write to screen (DI) and increment DI | ||
| + | jmp short X ; next pixel</syntaxhighlight> | ||
Revision as of 13:27, 15 August 2016
The FPU offers a lot of operations not available to classic x86 CPU, like SIN, COS, TAN, EXP, SQRT, LN and so on. Usage and communication with the FPU is a bit uncommon and takes a bit to get used to. It's recommended to read the creation of the snippet we want to modify first, this is how it 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 it 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 AH
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
The usual interaction with the FPU is as follows
-
F(N)INIT: Initialization of the FPU - store register content in memory location(s)
- transfer from memory location onto FPU stack
- actual calculations on the FPU (more on this soon)
- transfer from FPU stack into memory location(s)
- get register from memory location
That would be a lot for a single integer addition, but once more complex floating point operations are involved, it starts to pay off. 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 screenpointer 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] ;
stosb ; write to screen (DI) and increment DI
jmp short X ; next pixel
