[msdos][question] Which registers to preserve when calling an Assembly routine from C? #347

.define _plot
_plot:
    push    bp
    mov     bp, sp
    push    ax
    push    bx
    push    cx
    push    dx
    push    di
    
    ! mov bx, sp
    mov cx, 4(bp)
    mov dx, 6(bp)

    ! Set ES = A000h (VGA memory)
    mov     ax, 0xA000
    mov     es, ax

    ! Compute offset = y*320 + x
    mov     ax, dx       ! AX = y
    mov     bx, 320
    mul     bx           ! AX = y*320   (fits in 16 bits)
    add     ax, cx       ! AX = y*320 + x
    mov     di, ax       ! DI = offset

    movb al, 8(bp)       ! color

    ! Write pixel using STOSB
    stosb                 ! [ES:DI] = AL, increments DI

    pop     di
    pop     dx
    pop     cx
    pop     bx
    pop     ax
    pop     bp
    ret

extern void plot(uint16_t x, uint16_t y, uint8_t color);