Assembly 80386 sine with TASM and DOSBox

Question

I have to write a program that shows some kind of (possibly) complex shape that rotates and moves around. The problem is, for any kind of solution I can think of, I need the sine and cosine function.

First, I had a problem of setting to 386 mode too early (the executable should be 16 bit, compiled with TASM).

Right now the problem is that I get weird values for my loop:

.data

_180 dw 180
_25 dw 25
degrad dd ?
tempvar dw ?
alpha dw ?

.386
.code

start:

finit

fldpi
fidiv word ptr _180
fstp dword ptr degrad

    mov word ptr alpha, 0

calcsin:
mov ax, word ptr alpha
mov word ptr tempvar, ax
    fild tempvar
    fsin
    fimul word ptr _25
    fistp word ptr tempvar
mov DX,word ptr tempvar


inc word ptr alpha
cmp word ptr alpha,360
jne calcsin

    mov ax, 4c00h
    int 21h
end start

after this if I open up Turbo Debugger in DOSBox, I get weird, huge values in DX. I've also tried copying (and fixing, since they wouldn't compile) some examples from online to see if they work, they usually ended up not working...

Answer 1

You didn't initialize at least DS at the beginning of your code. Look:

.MODEL small
.486

.STACK 1000h

.DATA
    _180 dw 180
    _25 dw 25
    degrad dd ?
    tempvar dw ?
    alpha dw ?
    buf dw 8 dup (?)

.CODE
start PROC
    mov ax, @data
    mov ds, ax
    mov es, ax

    finit
    fldpi
    fidiv word ptr _180
    fstp dword ptr degrad
    mov word ptr alpha, 0

    calcsin:
    mov ax, word ptr alpha
    mov word ptr tempvar, ax
    fild tempvar
    fsin
    fimul word ptr _25
    fistp word ptr tempvar

    mov dx, word ptr tempvar

    mov ax, tempvar
    mov di, OFFSET buf
    call int2dez
    mov dx, OFFSET buf
    mov ah, 09h
    int 21h

    inc word ptr alpha
    cmp word ptr alpha, 360
    jne calcsin

    mov ax, 4C00h       ; Exit(0)
    int 21h
start ENDP

int2dez PROC
    test ax, 10000000b
    jz Convert
    mov byte ptr [di], '-'
    inc di
    neg ax

    Convert:
    mov bx, 10
    xor cx, cx
  Loop_1:
    xor dx, dx
    div bx
    push dx
    add cl, 1
    or  ax, ax
    jnz Loop_1
  Loop_2:
    pop ax
    or ax, 00110000b
    stosb
    loop Loop_2

    mov al, 10
    stosb
    mov al, '$'
    stosb
    ret
int2dez ENDP

end start

The program works but I didn't check the calculation and the results. I guess a rotation without sine (http://en.wikipedia.org/wiki/Midpoint_circle_algorithm) is quicker.

Answer 2

You forgot to convert from degrees to radians in your loop. Before fsin, place an fmul dword ptr degrad.