Category:ARM Assembly: Difference between revisions
→Limitations of the ARM
Puppydrum64 (talk | contribs) |
Puppydrum64 (talk | contribs) |
||
Line 177:
This isn't actually valid ARM code, it's more of a built-in macro. Essentially, the value will be loaded in one go as an immediate if it can. If not, it will get placed nearby as a data block and the <code>MOV</code> will be changed to an <code>LDR</code> command. Basically you can take everything in the above paragraph and forget about it, since equals notation does the work for you.
===THUMB Mode===
THUMB Mode is a more limited version of the ARM instruction set. The advantage to using it is that each instruction only takes 16 bits to represent rather than 32. This makes it handy for programming on systems that have very little space to work with. It can do almost anything 32-bit ARM can do, but not as easily. There are a few key limitations:
* Immediate operands can only be 8-bit values, period. In other words, only numbers ranging from 0 to 255 are allowed.
* You can still use LDR and ADR to retrieve embedded constants; however they have to be "further along" in memory than the current value of the program counter. In THUMB mode the program counter offsets cannot be negative.
* Registers R0-R7 can do almost anything, but registers of a higher number are harder to use. For registers R8 and above, you can no longer store immediate values into them, for example - you have to load them from registers.
* In THUMB mode you cannot use the barrel shifter, nor can you conditionally set the flags. THUMB Mode works more like an x86 CPU, where each instruction affects the flags differently (or sometimes not at all), and you just have to know which instructions affect which flags.
* Operations you would normally use the barrel shifter for are now separate commands. (You might be used to using these even in 32-bit ARM mode thanks to unified syntax.)
* The stack can still be interacted with using <code>PUSH</code> and <code>POP</code> (again, you were likely doing this anyway.)
That being said, it's not all doom and gloom. The registers are still 32-bit, and you can still do most of what the 32-bit ARM can do. If you're coding in C or some other language that gets compiled to ARM Assembly, the compiler will decide whether to use THUMB or 32-bit ARM, but you can request one or the other with command line arguments.
[[Category:Assembly]]
| |||