The CBW, CWDE, and CDQE instructions in x86 and x64 assembly double the size of the source operand. The CBW or Convert Byte to Word instruction sign extends the content of the AL register to the AX register. The CWDE or Convert Word to Doubleword instruction sign extends the content of the AX register to the EAX register. And, the CDQE or Convert Doubleword to Quadword instruction sign extends the content of the EAX register to the RAX register in x64.

When we sign-extend the content of the AL register, the sign-bit or bit 7 is copied into every bit of the AH register. Similarly, the content of the AX or EAX register is sign-extended in CWDE or CDQE instruction respectively.

Please note that no flag is affected during the execution of CBW, CWDE, or CDQE instruction.

Where are CBW, CWDE, and CDQE instructions used in x86 and x64 assembly?

Let’s say we are using a string or array of characters. And, we want to access each element of the array using an index. In that case, the integer index variable maybe first read into a register, say the EAX register and then, the content of the EAX register can be sign-extended to the RAX register using the cdqe instruction. After that, the sign-extended value can be used, e.g. “mov BYTE PTR [rbp+rax*1-0x10],dl” to access (here to assign the content of the DL register to the array element) each element of the array. Please note that here the first element of the array is referenced by [RBP – 0x10] and initially the RAX register contains 0x1. We can then increment the content of the RAX register to access each element of the array.