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CBW
Extends the sign of the AL register to the AX register. The value of AL is sign-extended into AH.
The following table covers what the source and destinations can be:
| source | destination(s) |
|---|---|
| reg (AL) | reg (AX) |
| imm | #I |
| memory | #I |
DO NOT support LOCK
The instruction is available in 64-bit mode and compatibility mode. It operates specifically on the low byte of the AX register; therefore, it cannot be used with any other register.
The instruction implicitly targets AL and AX. Attempting to use CBW in a context where the AX register is not accessible or is restricted by the current operating mode SHALL result in an invalid operation. Since this is a sign-extension operation, the most significant bit of AL (bit 7) determines whether AH is set to 00h or FFh.