Using Division Instructions:
The DIV instruction divides unsigned numbers, and IDIV divides
signed numbers. Both return a quotient and a remainder.
Table 4.1 summarizes the division operations. The dividend
is the number to be divided, and the divisor is the number to
divide by. The quotient is the result. The divisor can be in any
register or memory location but cannot be an immediate value.
|Size of Operand ||Dividend Register ||Size of Divisor ||Quotient ||Remainder
|16 bits ||AX ||8 bits ||AL ||AH
|32 bits ||DX:AX ||16 bits ||AX ||DX
|64 bits (80386and 80486)||EDX:EAX ||32 bits ||EAX ||EDX
Table: Division Operations
Unsigned division does not require careful attention to flags.
The following examples illustrate signed division, which can be more complex.
mem16 SWORD -2000
mem32 SDWORD 500000
; Divide 16-bit unsigned by 8-bit
mov ax, 700 ; Load dividend 700
mov bl, 36 ; Load divisor DIV 36
div bl ; Divide BL ------
; Quotient in AL 19
; Remainder in AH 16
; Divide 32-bit signed by 16-bit
mov ax, WORD PTR mem32 ; Load into DX:AX
mov dx, WORD PTR mem32 ; 500000
idiv mem16 ; DIV -2000
; Divide memory ------
; Quotient in AX -250
; Remainder in DX 0
; Divide 16-bit signed by 16-bit
mov ax, WORD PTR mem16 ; Load into AX -2000
cwd ; Extend to DX:AX
mov bx,-421 ; DIV -421
idiv bx ; Divide by BX -----
; Quotient in AX 4
; Remainder in DX -316
If the dividend and divisor are the same size, sign-extend or
zero-extend the dividend so that it is the length expected by
the division instruction. See “Extending Signed and Unsigned
Integers,” later in this unit.