- The add instruction can operate with any register
add DEST, SRC
Effect:
- DEST = DEST + SRC
Note:
- Make sure that the size of the source and destination operands are equal
- Examples:
i dd 89 add eax, 12 add eax, ebx add eax, i
- The sub instruction can operate with any register
sub DEST, SRC
Effect:
- DEST = DEST - SRC
Note:
- Make sure that the size of the source and destination operands are equal
- Examples:
i dd 89 sub eax, 12 sub eax, ebx sub eax, i
- Signed and unsigned multiplication:
- imul =
signed multiplication
(we only learned to use signed multiplication in M68000 and SPARC)
- mul = unsigned multiplication
- imul =
signed multiplication
- Note:
- There are many formats
of the multiply instruction
- I will only discuss the single operand
format.
(This format works in all versions of Intel CPUs, even in 8080 :-))
- There are many formats
of the multiply instruction
- Imul syntax: (for the single operand imul)
imul SRC
Effect:
DEST = DEST * SRC DEST is some part of the EAX register Which part will depend on the SRC operand
Byte size SRC operand: AX = AL * SRC(8) Word size SRC operand: DX:AX = AX * SRC(16) Double word size SRC operand: EDX:EAX = EAX * SRC(32)
- Example: EDX = BX (16 bit value) * AX (16 bit value)
mov ax, 10000 mov bx, 8888 imul bx ;; Result DX:AX = ax * bx ;; DX AX ;; +---------+ +---------+ ;; |xxxxxxxxx| |yyyyyyyyy| ;; +---------+ +---------+ ;; ROL EDX, 16 ;; Rotate EDX 16 bits to the LEFT ;; ;; Before: ;; EDX ;; +---------+---------+ ;; |?????????|xxxxxxxxx| ;; +---------+---------+ ;; ;; After: ;; EDX ;; +---------+---------+ ;; |xxxxxxxxx|?????????| ;; +---------+---------+ ;; ;; This will put lower EDX in upper EDX mov dx, ax ;; Result: ;; EDX ;; +---------+---------+ ;; |xxxxxxxxx|yyyyyyyyy| ;; +---------+---------+
- Signed and unsigned division:
- idiv =
signed division
(we only learned to use signed division in M68000 and SPARC)
- div = unsigned division
- idiv =
signed division
- Note:
- There are many formats
of the divide instruction
- I will only discuss the single operand
format.
(This format works in all versions of Intel CPUs, even in 8080 :-))
- There are many formats
of the divide instruction
- Idiv syntax: (for the single operand imul)
idiv SRC
Effect:
- Just like the
M68000 divide instruction,
the idvi instruction
computes
the quotient
and the remainder
- The
destination register
of the divide instruction is
always
the accumulator
- Effect for different SRC operand sizes
idiv SRC ---> DEST = DEST / SRC Remainder: Quotient Byte size SRC: AH : AL = AX / SRC(8) Word size SRC: DX : AX = DX:AX / SRC(16) Double word size SRC: EDX : EAX = EDX:EAX / SRC(32)
-----------------+-----------------+-------------+---------+--------- |Must use this | | | Size of Operand |Dividend Register| Size Divisor| Quotient|Remainder -----------------+-----------------+-------------+---------+--------- 16 bits | AX | 8 bits | AL | AH -----------------+-----------------+-------------+---------+--------- 32 bits | DX:AX | 16 bits | AX | DX -----------------+-----------------+-------------+---------+--------- 64 bits | EDX:EAX | 32 bits | EAX | EDX -----------------+-----------------+-------------+---------+---------
- Just like the
M68000 divide instruction,
the idvi instruction
computes
the quotient
and the remainder
- Example:
mov ax, 18 ;; AX(16 bits) = 18 cwd ;; DX:AX(32 bits) = 18 mov bx, 7 ;; Prepare for division idiv bx ;; Divide by 16 bit source... ;; ;; Operation performed: ;; DX:AX / BX ;; ;; Result: ;; AX = quotient ;; DX = remainder
- Prog file: click here