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Safe to use r13-r15 as general register?

Note: This was originally posted on 18th July 2011 at http://forums.arm.com

Hello, I am writting a function, which involves complicated algorithm, I want to have a lot of general purpose registers to keep the routine speed. I have read some article that r13-r15 are special registers, but can be used as general purpose registers.

Can I do it like this
stmfd sp!, {r4-r15,lr}
:
:
ldmfd sp!, {r4-r15,pc}

Thank you very much!

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Etienne SOBOLE over 12 years ago

Note: This was originally posted on 19th July 2011 at http://forums.arm.com

You can use r13 and r14.
I used to do that

To save and restore register you can do this:

my_fct: movw r12, #:lower16:.save_zone movt r12, #:upper16:.save_zone stmia r12, {r4 - r11, sp, lr} ... movw r12, #:lower16:.save_zone movt r12, #:upper16:.save_zone ldmia r12, {r4 - r11, sp, pc} .data .save_zone: .word 0, 0, 0, 0, 0, 0, 0 .word 0, 0, 0, 0, 0, 0, 0

r12 can be used because it is a scratch register, but if you want, you can push it before changing it and restore it after having restored sp (r13) !

Etienne
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Etienne SOBOLE over 12 years ago

Note: This was originally posted on 19th July 2011 at http://forums.arm.com

You can use r13 and r14.
I used to do that

To save and restore register you can do this:

my_fct: movw r12, #:lower16:.save_zone movt r12, #:upper16:.save_zone stmia r12, {r4 - r11, sp, lr} ... movw r12, #:lower16:.save_zone movt r12, #:upper16:.save_zone ldmia r12, {r4 - r11, sp, pc} .data .save_zone: .word 0, 0, 0, 0, 0, 0, 0 .word 0, 0, 0, 0, 0, 0, 0

r12 can be used because it is a scratch register, but if you want, you can push it before changing it and restore it after having restored sp (r13) !

Etienne
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