what is the scope of assembly code registers?-CodePudding

I have a piece of assembly code of a function in a program I am analysing.

000081e4 <_ZN7CamTask7executeEv>:
81e4:   b580        push    {r7, lr}
81e6:   b084        sub sp, #16
81e8:   af00        add r7, sp, #0
81ea:   6078        str r0, [r7, #4]
81ec:   230a        movs    r3, #10
81ee:   60fb        str r3, [r7, #12]
81f0:   687b        ldr r3, [r7, #4]
81f2:   6ddb        ldr r3, [r3, #92]   ; 0x5c
81f4:   68f9        ldr r1, [r7, #12]
81f6:   4618        mov r0, r3
81f8:   f7ff ffb7   bl  816a <_ZN10ImgChannel9pushValueEi>
81fc:   bf00        nop
81fe:   3710        adds    r7, #16
8200:   46bd        mov sp, r7
8202:   bd80        pop {r7, pc}

1 - is the scope of registers inside a function of assembly limited to that function for example the 4th line here 81ea: 6078 str r0, [r7, #4] where did the value inside r0 come from ?.was it passed as an argument by another function that calls this function?. If so is it also called r0 before it is passed?

for example the line bl 816a <_ZN10ImgChannel9pushValueEi> in the above function <_ZN7CamTask7executeEv> calls function _ZN10ImgChannel9pushValueEi below ..is r0 in line 81f6: 4618 mov r0, r3 in the above function the same as r0 in line 8170: 6078 str r0, [r7, #4] in the below function.

0000816a <_ZN10ImgChannel9pushValueEi>:
816a:   b580        push    {r7, lr}
816c:   b082        sub sp, #8
816e:   af00        add r7, sp, #0
8170:   6078        str r0, [r7, #4]
8172:   6039        str r1, [r7, #0]
8174:   687b        ldr r3, [r7, #4]
8176:   683a        ldr r2, [r7, #0]
8178:   60da        str r2, [r3, #12]
817a:   687b        ldr r3, [r7, #4]
817c:   4618        mov r0, r3
817e:   f001 ff06   bl  9f8e <_ZN7Tasking7Channel4pushEv>
8182:   bf00        nop
8184:   3708        adds    r7, #8
8186:   46bd        mov sp, r7
8188:   bd80        pop {r7, pc}

2- Can the below function change the value of r0 and then return to the calling function with r0 being updated now?

CodePudding user response：

The register usage is set forth by the calling convention, which is part of the ABI. Calling conventions are very detailed in how parameters are passed, return values are returned, and which registers can be used with/without preservation. See here for more information about calling conventions. (Search for ARM.)

was it passed as an argument by another function that calls this function?

Yes

If so is it also called r0 before it is passed?

The registers are physical storage and they are always accessible to any machine code — there is no scope, but rather a sharing agreement between functions. We can think of registers as permanent physical storage, always availability, and globally accessible.

Can the below function change the value of r0 and then return to the calling function with r0 being updated now?

Yes, for simple integer and pointer arguments, functions take their first parameter in r0 (next in r1) and return values in r0 (and r1 as well if longer).