Arm assembly study notes (eight)-the use of registers

• In the program, we can use 14 of the 16 general registers to save our own data, r13 and r15 are sp and pc registers, we can not use.

• ATPCS specifies that the stack pointer should be 8-byte aligned.

• ATPCS stipulates that the called function must ensure that the contents of r4-r11 remain unchanged before and after execution, that is, the called function must save r4-r11 before starting and restore it before returning. As shown below:

You can use the following save template:

routine_name
                    STMFD sp!, {r4-r12, lr} ; stack saved registers
                    ; body of routine
                    ; the fourteen registers r0-r12 and lr are available
                    LDMFD sp!, {r4-r12, pc} ; restore registers and return

The r12 is saved for 8-byte stack pointer alignment.

• ATPCS stipulates that during function calls, r0-r3 are used to pass parameters, and more than 4 parameters are saved in the stack. The return value must be stored in r0.

• For multi-register push and pop instructions, such as LDM and STM, the register numbers in the register list must be in ascending order.

• Double-word load and store instructions, LDRD and STRD, operand registers are two adjacent registers, Rd and Rd + 1, and the register number of this Rd must be an even number.
• Use register names rather than physical register numbers when writing assembly routines. This makes it easier to reallocate registers and to maintain the code.
• In order to save registers, we can store some variables with relatively few digits in a register, and then we can take out the corresponding variables through the shift operation. See the example below:

sample = table[index];
index += increment;

Under normal circumstances, index and increment will not exceed 16 bits, we can store them in the same register, as shown below

The above C code can be implemented with the following assembly code:

LDRB sample, [table, indinc, LSR#16] ; table[index]
ADD indinc, indinc, indinc, LSL#16 ; index+=increment