Division and implicit register pairs

The idea

For a 32-bit DIV operand, unsigned division uses the 64-bit dividend EDX:EAX and returns quotient in EAX and remainder in EDX. Setting only EAX is insufficient if EDX contains unrelated bits. To divide an ordinary unsigned 32-bit value, clear EDX first. The divisor must be nonzero, and the quotient must fit the destination width.

IDIV performs signed division and truncates the quotient toward zero. For a signed 32-bit dividend in EAX, CDQ sign-extends into EDX:EAX before IDIV. The remainder has the dividend’s sign or is zero. Dividing by zero or producing an unrepresentable quotient causes a divide exception; arithmetic flags are not the mechanism for checking those outcomes afterward. These are conceptual traces, not standalone executable browser programs.

; Signed 32-bit division: -17 / 5
mov eax, -17
cdq
mov ecx, 5
idiv ecx

EAX = -3; EDX = -2 as signed 32-bit values.

The quotient truncates toward zero, and -17 = (-3)*5 + (-2).