Assembly 8086 EQU directive

Question

I'm having trouble just making clear EQU directive in assembler (8086).

abc EQU xyz

Does EQU literally swaps abc when found in code with xyz, whatever xyz represents, value etc?

i.e. Can i write?

varA EQU [bp+4]

mov ax, varA

And one more question is EQU globally accessed, i.e. can I define EQU out of procedure, and in procedure to use it?

Answer 1

EQU items are not variables, they don't take any memory space :

• When EQU referes to a constant value, it becomes a synonym for that value. This value can't be overwritten, even if you try it won't change.
• When EQU referes to another variable, it becomes a synonym for that variable, so everything that happens to the synonym will happen to the variable.

Copy-paste next code in EMU8086 and run :

.model small
.stack 100h
.data

xyz DW  2016    ;◄■■■ ABC IS NOT A VARIABLE, IT IS
abc EQU xyz     ;     JUST A SYNONYM FOR XYZ.

pqr EQU 10      ;◄■■■ PQR IS NOT A VARIABLE, IT IS
                ;     JUST A SNYNONYM FOR NUMBER 10.

varA EQU [bp+2] ;◄■■■ BP POINTS TO GARBAGE.

.code

mov  ax, @data
mov  ds, ax

mov  abc, 25    ;◄■■■ XYZ BECOMES 25!!!!

mov  pqr, 999   ;◄■■■ NO ERROR, BUT THE VALUE WILL NOT CHANGE.
mov  ax, pqr    ;◄■■■ AX IS NOT 999, AX=10.

mov  si, varA   ;◄■■■ GARBAGE.
mov  bp, sp
mov  si, varA   ;◄■■■ DIFFERENT GARBAGE.
push ax         ;◄■■■ PUSH 10.
call my_proc

mov  ax, NUMBER ;◄■■■ YES, EQUS ARE GLOBAL!!! (AX=0B9H).

mov  ax, 4c00h
int  21h

;-----------------------------------------

my_proc proc  
mov  bp, sp
mov  si, varA    ;◄■■■ WRONG VALUE (ANOTHER GARBAGE). 
mov  si, [bp+2]  ;◄■■■ PROPER VALUE (10).

varB EQU [bp+2]
mov  si, varB    ;◄■■■ WRONG AGAIN.

NUMBER EQU 0b9h  ;◄■■■ DEFINE EQU INSIDE PROCEDURE.

ret
my_proc endp          

In the case of [bp+2] it just doesn't seem to work, probably because the compiler can't get a fixed value.