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DB - Define Byte. 8 bits DW - Define Word. Generally 2 bytes on a typical x86 32-bit system DD - Define double word. Generally 4 bytes on a typical x86 32-bit system From x86 assembly tutorial, The pop instruction removes the 4-byte data element from the top of the hardware-supported stack into the specified operand (i.e. register or memory location). It first moves the 4 bytes located at memory location [SP] into the specified register or memory location, and then increments SP by 4. Your num is 1 byte. Try declaring it with DD so that it becomes 4 bytes and matches with pop semantics.