OR

OR combines the two operands bit by bit and stores the result in the first. A result bit is 1 when either operand (or both) has a 1 there. Used with a mask, OR switches on the bits where the mask has a 1 and does not touch the others.

   0100 0001    AL = 41h ("A")
OR 0010 0000    mask 20h
 = 0110 0001    AL = 61h ("a")

Forms and flags

The operand rules are the same as for AND: register or memory destination, register, memory or immediate source, equal sizes, no memory-to-memory, no segment registers. CF and OF are cleared, SF ZF PF follow the result, AF is undefined.

Idioms

Set a bit in a hardware register or a flag byte: OR AL, 3 turns on bits 0 and 1 (the guide's speaker example does this with port 61h). Make a digit printable: values 0..9 OR 30h give "0".."9". Lowercase an ASCII letter: OR AL, 20h. Glue two parts together once they sit in different bits, for example a high nibble already shifted into place OR a low nibble.

An old habit is OR AX, AX to check a register: the value does not change, but ZF and SF now say whether it is zero or negative. TEST AX, AX does the same and states the intent more clearly.

MOV     AL, 41h       ; 0100 0001 = "A"
CALL    SHOWBIN
OR      AL, 20h       ; set bit 5
CALL    SHOWBIN       ; 0110 0001
MOV     DL, AL
MOV     AH, 02h
INT     21h           ; prints "a"
MOV     AL, 7
OR      AL, 30h       ; 7 -> "7"
MOV     DL, AL
MOV     AH, 02h
INT     21h
 
SHOWBIN PROC    NEAR  ; print AL as 8 binary digits and a space
PUSHF                 ; keep the caller's flags
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BL, AL
MOV     CX, 8
NEXTBIT:
SHL     BL, 1         ; the top bit falls into CF
MOV     DL, "0"
ADC     DL, 0         ; "0" + CF = "0" or "1"
MOV     AH, 02h
INT     21h
LOOP    NEXTBIT
MOV     DL, " "
MOV     AH, 02h
INT     21h
POP     DX
POP     CX
POP     BX
POP     AX
POPF
RET
SHOWBIN ENDP