最近在看 intel 指令手册，有一处不明，求解。

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In 64-bit mode, there are limitations on accessing byte registers. An instruction cannot reference legacy high-bytes (for example: AH, BH, CH, DH) and one of the new byte registers at the same time (for example: the low byte of the RAX register)

这段话该怎么理解？
什么是one of the new byte registers？
谢谢！

byte

registers

accessing

9 replies • 2015-02-01 23:18:18 +08:00

yangff

Feb 1, 2015 via Android

ah bh ch ...是高位啊。。。

hazard

Feb 1, 2015

@yangff 那 one of the new byte registers 指的是什么？是AL吗？

xieyudi1990

Feb 1, 2015

感觉 "new byte registers" 指的是 AL, BL, CL, DL, DIL, SIL, BPL, SPL, R8L - R15L.
意思应该是 AH BH CH DH 和上面这些不能同时在一条指令里引用.

没玩过x86_64, 但我翻了下手册, 结合上下文应该是这个意思.

In 64-bit mode, there are limitations on accessing byte registers. An instruction cannot reference legacy high-bytes (for example: AH, BH, CH, DH) and one of the new byte registers at the same time (for example: the low byte of the RAX register). However, instructions may reference legacy low-bytes (for example: AL, BL, CL or DL) and new byte registers at the same time (for example: the low byte of the R8 register, or RBP). The architecture enforces this limitation by changing high-byte references (AH, BH, CH, DH) to low byte references (BPL, SPL, DIL, SIL: the low 8 bits for RBP, RSP, RDI and RSI) for instructions using a REX prefix.

xieyudi1990

Feb 1, 2015

Byte Registers: AL, BL, CL, DL, DIL, SIL, BPL, SPL, R8L - R15L

bcxx

Feb 1, 2015

@hazard 指 r 开头的那些寄存器吧, 这里的意思应该是指在 64 位下的那些 extended registier 也是只能访问到一个字节(我乱说的,别当真) 应该是和寄存器拓展长度有关的. http://stackoverflow.com/a/1753618

yfdyh000

Feb 1, 2015

在64位模式，将限制指令存取字节寄存器，指令不能同时使用传统的高字节（比如AH, BH. CH, DH）和新的字节寄存器（比如RAX寄存器的低字节）。-- http://baike.baidu.com/view/809.htm

yangff

Feb 1, 2015 via Android

头搞晕。。
反正就是r**那票64位的寄存器的低位不能和他们的高位一起ref。。

yangff

Feb 1, 2015 via Android

阿不是。。r**那票的低位

hazard

Feb 1, 2015

明白了谢谢各位！