如何在具有立即数作为第二个操作数的ARM中使用MOV指令 [英] How to use MOV instruction in ARM with an immediate number as the second operand
问题描述
我只是开始学习ARM汇编语言，还不清楚如何使用MOV将立即数传输到寄存器中.
I just begin to study ARM assembly language, and am not clear about how to use MOV to transfer an immediate number into a register.
从ARM参考手册和我的教科书中都可以看出，MOV指令后的立即数范围是0-255.但是，当我在ADS 1.2 IDE中在自己的PC上进行测试时，说明
From both the ARM reference manual and my textbook, it's said that range of immediate number following MOV instruction is 0-255. But when I test on my own PC in ADS 1.2 IDE, instruction
MOV R2, #0xFFFFFFFF
表现良好.数字0xFFFFFFFF是否超出规范范围?
performs well. Isn't number 0xFFFFFFFF out of range according to the specification?
希望有人可以帮我
致谢.
推荐答案
请记住，作为合并到ARM操作码中的桶形移位器的一部分，ARM可以对立即数执行一组特定的操作.
Remember that the ARM can perform a certain set of manipulations on the immediate value as part of the barrel shifter that is incorporated into the ARM's opcodes.
这篇小文章对ARM汇编程序可用于将大量立即数放入ARM指令的较小可用空间中的一些技巧进行了最清晰的解释:
This little article has one of the clearest explanations of some of the tricks that an ARM assembler can use to fit a large immediate number into the small available space of an ARM instruction:
本文讨论了在生成MVN操作码以加载即时值的按位补码的特定示例中可能使用的技巧.
The article discusses the trick likely used in your specific example of generating a MVN opcode to load the bitwise complement of the immediate value.
无法用所有立即数完成这些类型的操作，但是ARM汇编程序据称非常聪明(C编译器当然可以).如果无法执行移位/补码技巧，则通常将从相对于PC的位置加载该值，或者可能通过从几条指令中累积"该值来加载.
These kinds of manipulation can't be done with all immediate values, but the ARM assemblers are supposedly pretty smart about it (and C compilers certainly are). If no shift/complement tricks can be performed, the value will generally be loaded from a PC-relative location or maybe by 'building up' the value from several instructions.
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