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;

;       Optional Assembly bitmask code.

;       This is only used if you #define USE_ASM in ire_game

;

;       Written more for amusement than for speed :-)

;



[BITS 32]



[global _SetBit_ASM]

[global _ResetBit_ASM]

[global _GetBit_ASM]



[EXTERN _cmap_width]

[EXTERN _cmap_height]



[SECTION .text]



;

;  Set a bit in the collision matrix

;



_SetBit_ASM:

    push ebp

    mov ebp,esp

    pushad



    mov eax, [_cmap_width]

    mov edi, [ebp+16]           ; EDI is the collision matrix

    mov edx, [ebp+12]           ; Get Y coordinate

    mov ebx, [ebp+8]            ; Get X coordinate



    mul edx                     ; Get offset, Y*width

    add edi, eax                ; EDI now points to the right byte column



    mov ecx, ebx                ; Save the X coordinate in CX

    shr ebx, 3                  ; Divide by 8, to get the byte

    add edi, ebx                ; Add the X coordinate



    and cl,  7                  ; Get the bit offset, 0-7 from the remainder

    mov al,  1                  ; Set AL to be one, and shift it by CL

    shl al,  cl                 ; Now we have the bitmask for ORing

    or  [edi], al               ; Set the bit we want



    popad

    pop ebp

    ret



;

;  Reset a bit in the collision matrix

;



_ResetBit_ASM:

    push ebp

    mov ebp,esp

    pushad



    mov eax, [_cmap_width]

    mov edi, [ebp+16]           ; EDI is the collision matrix

    mov edx, [ebp+12]           ; Get Y coordinate

    mov ebx, [ebp+8]            ; Get X coordinate



    mul edx                     ; Get offset, Y*width

    add edi, eax                ; EDI now points to the right byte column



    mov ecx, ebx                ; Save the X coordinate in CX

    shr ebx, 3                  ; Divide by 8, to get the byte

    add edi, ebx                ; Add the X coordinate



    and cl,  7                  ; Get the bit offset, 0-7 from the remainder

    mov al,  1                  ; Set AL to be one, and shift it by CL

    shl al,  cl                 ; Now we have the bitmask for ORing

    xor al,255                  ; Invert it

    and  [edi], al              ; Set the bit we want



    popad

    pop ebp

    ret



;

;  Get the state of an element in the collision matrix

;



_GetBit_ASM:

    push ebp

    mov ebp,esp



    xor eax,eax                 ; Zero EAX for the return



    pushad

    mov edi, [ebp+16]           ; EDI is the collision matrix

    mov eax, [_cmap_width]

    mov edx, [ebp+12]           ; Get Y coordinate

    mov ebx, [ebp+8]            ; Get X coordinate



    mul edx                     ; Get offset, Y*width

    add edi, eax                ; EDI now points to the right byte column



    mov ecx, ebx                ; Save the X coordinate in CX

    shr ebx, 3                  ; Divide by 8, to get the byte

    add edi, ebx                ; Add the X coordinate



    and cx,  7                  ; Get the bit offset, 0-7 from the remainder



    mov al,  [edi]              ; Copy the interesting byte into al

    btc ax,cx			; Get the contents of bit CL in byte AL

                                ; Carry Flag is now set to this value

    popad                       ; EAX returns to zero

    pop ebp

    salc			; SET AL=CARRY - http://www.x86.org

    ret                         ; SALC is undocumented but present in all

                                ; known x86 compatible microprocessors