Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
921 views
in Technique[技术] by (71.8m points)

assembly - How can ARM's MOV instruction work with a large number as the second operand?

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.

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

performs well. Isn't number 0xFFFFFFFF out of range according to the specification?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

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.

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:

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.

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.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...