MSP430 中的 sxt 指令到底有什么作用？

Question

I understand that sxt is sign extend instruction. But I don't understand how does this really work. For example if my r15 register stores byte 0045 , then what sxt r15 would do to it?

Answer 1

I've been working on problems that use this instruction a lot. To compress it down to a single function, I would write it (using Pythonic syntax) as:

def sxt(x):
    if x % 256 > 127:    # if top bit of low byte is set
        x |= 0xff00
    else:
        x &= 0x00ff

This could be written more compactly, and possibly more efficiently. This version is intended to be most intuitive for non-asm-natives.

Note also that the function assumes you pass inputs in as two byte words like the sxt instruction operates on, 0xhhhh where h is a hex digit, which you can do in Python.

Answer 2

The sxt instruction sign extends a byte into a 16 bit word, ie it copies bit 7 into bits 8 to 15 and then updates the flags appropriately.

For your example, sxt would have no effect as the byte 0045 does not have its most significant bit set, so the high byte of r15 stays at zero.