The FPU offers a lot of operations not available to classic x86 CPU, like
LN and so on. SIMPLY FPU by Raymond Filiatreault has a compact overview of all FPU commands. 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 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 this access memory in regard to the segment register
SS(see here, at 220.127.116.11 The 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"