Slide 15.7: IDIV instruction (cont.)

IDIV Instruction (Cont.)

The CWD (convert word to doubleword) instruction extends the sign bit of AX into the DX register.
The CDQ (convert doubleword to quadword) instruction extends the sign bit of EAX into the EDX register.

CWD — Convert Word to Doubleword

Usage: cwd

Flag	O	D	I	S	Z	A	P	C
Result

Extends sign of word in register AX throughout register DX forming a doubleword quantity in DX:AX.

CDQ — Convert Doubleword to Quadword

Usage: cdq

Flag	O	D	I	S	Z	A	P	C
Result

Converts signed DWORD in EAX to a signed quad word in EDX:EAX by extending the high order bit of EAX throughout EDX.

16-Bit Signed Division	32-Bit Signed Division	Using `CDQ`
.data x SDWORD 00101010h y SWORD -100h .code mov dx, WORD PTR x+2 movzx eax, WORD PTR x idiv y call DumpRegs	.data x SDWORD -101010h y SDWORD 100h .code mov eax, x mov edx, 0FFFFFFFFh idiv y call DumpRegs	.data x SDWORD -101010h y SDWORD 100h .code mov eax, x cdq idiv y call DumpRegs
Output	Output	Output
AX = DX =	EAX = EDX =	EAX = EDX =