::::::. ::/ \:::. :/___\::::. /| \:::::. :| _/\::::::. :| _|\ \:::::::. :::\_____\::::::::. ::::::::::::::::::::. ::::::::::::::::::::::.

Assembly Language Snippets: Abs
        mov     ecx, eax        ; Duplicate value
        sar     ecx, 31         ; Fill ecx with its sign
        xor     eax, ecx        ; Do 'not eax' if negative
        sub     eax, ecx        ; Do 'inc eax' if negative

Extending NASM
;===========================================================-SWITCH-CASE Blocks
%macro SWITCH 1
%push switch
%assign __curr 1
    mov eax, %1
    jmp %$loc(__curr)
%endmacro

%macro CASE 1
%ifctx switch
    %$loc(__curr):
    %assign __curr __curr+1
    mov ebx, %1
    cmp eax, ebx
    jne %$loc(__curr)
%endif
%endmacro

%macro DEFAULT 0
%ifctx switch
    %$loc(__curr):
%endif
%endmacro

%macro BREAK 0
    jmp %$endswitch
%endmacro

%macro ENDSWITCH 0
    %ifctx switch
    %$endswitch:
    %pop
    %endif
%endmacro
;==========================================================================-END
The problem here is the %$loc(__curr);  even with brackets replacing the
parentheses, the code does not compile correctly in NASM 0.97.

< anybody> fixed these as part of the visasm project; here is his replacement
code:
;===========================================================-SWITCH-CASE Blocks

%macro SWITCH_MAC 1
%push switch
[section data]
%$var1  dw      0
%$var2  db      0

[section code]
%push case_clause

%ifidni %1,ax
        %define bit16
        mov word [%$$var1],ax
%elifidni %1,bx
        %define bit16
        mov word [%$$var1],bx
%elifidni %1,cx
        %define bit16
        mov word [%$$var1],cx
%elifidni %1,dx
        %define bit16
        mov word [%$$var1],dx
%elifidni %1,al
        %define bit8
        mov byte [%$$var2],al
%elifidni %1,bl
        %define bit8
        mov byte [%$$var2],bl
%elifidni %1,cl
        %define bit8
        mov byte [%$$var2],cl
%elifidni %1,dl
        %define bit8
        mov byte [%$$var2],dl
%elifidni %1,ah
        %define bit8
        mov byte [%$$var2],ah
%else
        mov eax, %1
%endif
%endmacro

%macro CASE_MAC 1
    %$case_code:
%pop
%push case_clause

%ifdef bit16
    cmp word [%$$var1],%1
%elifdef bit8
    cmp byte [%$$var2],%1
%else
    cmp eax, %1
%endif
    je  %%code
    jmp %$case_code
%%code:
%endmacro

%imacro DEFAULT 0
%define _default
    %$case_code:
%endmacro

%imacro BREAK 0
    jmp %$$endswitch
%endmacro

%imacro ENDSWITCH 0
%ifndef _default
    %$case_code:
%else
%undef _default
%endif
    %pop
    %$endswitch:
    %pop
    %undef bit16
    %undef bit8
%endmacro

%idefine SWITCH(var1) SWITCH_MAC var1
%idefine CASE(var1) CASE_MAC var1
;==========================================================================-END

Programming The VGA Graphics Mode with 13h
In the code supplied, a pixel is put into CX
from memory and the code which was meant to do so is this:

     mov     cx,es:[ax]      ; now pixel x,y can be...
This will not work (ax is not a legal index register for
memory addressing); changing the code as follows works:
     mov       si,ax             ; mov SI,AX for correct mem. addressing
     mov     cx,es:[si]      ; now pixel x,y can be accessed as es:[si]