AND

AND combines its two operands bit by bit and writes the result into the first one. A result bit is 1 only when the matching bit is 1 in both operands. Think of the second operand as a mask: where the mask has a 1 the bit survives, where it has a 0 the bit is forced to 0.

    1011 0110    AL = B6h
AND 0000 1111    mask 0Fh
  = 0000 0110    AL = 06h: only the low nibble is left

Forms

The destination is a register or a memory location; the source is a register, memory or an immediate value. Both must be the same size, byte or word. There is no memory-to-memory form (load one side into a register first), segment registers cannot be used, and an immediate cannot be the destination. With memory and an immediate, say the size: AND BYTE PTR [BX], 7Fh.

Flags

CF and OF are always cleared. SF, ZF and PF describe the result, so JZ right after an AND tells you that no masked bit was set. AF is undefined. If you only want the flags and need to keep the value, use TEST instead.

Idioms

Keep the low nibble before turning it into a digit: AND AL, 0Fh. Take the remainder of an unsigned division by a power of two: AND AX, 7 leaves AX mod 8. Round down to a multiple of 16: AND AX, 0FFF0h. Turn an ASCII lowercase letter into uppercase by clearing bit 5: AND AL, 0DFh (only for letters; other characters get changed too).

MOV     AL, 0B6h      ; 1011 0110
CALL    SHOWBIN
AND     AL, 0Fh       ; keep bits 0..3
CALL    SHOWBIN       ; 0000 0110
MOV     AL, "q"
AND     AL, 0DFh      ; clear bit 5: "Q"
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