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How do I use branching in a loop in ARM Assembly?
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My program creates a 2d array in memory with .skip 1000. It then populates that array with an input from stdin using this loop:
@@loop to store message in array
@outer for loop over rows
MOV r0,#0 @r0 = i (row index)
msgrowloop:
CMP r0,r2 @compare to nrows
BEQ msgendrowloop
@multiply/accumulate instruction
MLA r7, r3, r0, r6 @calculates the address of the first element in each row
@inner for loop over columns
MOV r1,#0 @r1 = j (column index)
msgcolumnloop:
CMP r1,r3 @compare to ncolumns
BEQ msgendcolumnloop
@@@store from stdin
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
@@@store from stdin end
@store r8 in memory and increase r7 by one byte
STRB r8,[r7],#1
ADD r1,r1,#1 @j += 1
B msgcolumnloop
msgendcolumnloop:
ADD r0,r0,#1 @i += 1
B msgrowloop
msgendrowloop:
@rest of the program...
Now, using this I get a segmentation error, but if I change my stdin function to this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0, #-1 @check if we are at end of file
MOV r8, r0 @move character to r8
POP {r0-r4}
BEQ msgendrowloop @exit loop when done
Instead of this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
It works perfectly. The logic here is confusing as my original code seems logically sound.
assembly raspberry-pi arm
edited 2 days ago
cooperised
1,042712
asked 2 days ago
eddiewastaken
1768
add a comment |
up vote
0
down vote
favorite
My program creates a 2d array in memory with .skip 1000. It then populates that array with an input from stdin using this loop:
@@loop to store message in array
@outer for loop over rows
MOV r0,#0 @r0 = i (row index)
msgrowloop:
CMP r0,r2 @compare to nrows
BEQ msgendrowloop
@multiply/accumulate instruction
MLA r7, r3, r0, r6 @calculates the address of the first element in each row
@inner for loop over columns
MOV r1,#0 @r1 = j (column index)
msgcolumnloop:
CMP r1,r3 @compare to ncolumns
BEQ msgendcolumnloop
@@@store from stdin
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
@@@store from stdin end
@store r8 in memory and increase r7 by one byte
STRB r8,[r7],#1
ADD r1,r1,#1 @j += 1
B msgcolumnloop
msgendcolumnloop:
ADD r0,r0,#1 @i += 1
B msgrowloop
msgendrowloop:
@rest of the program...
Now, using this I get a segmentation error, but if I change my stdin function to this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0, #-1 @check if we are at end of file
MOV r8, r0 @move character to r8
POP {r0-r4}
BEQ msgendrowloop @exit loop when done
Instead of this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
It works perfectly. The logic here is confusing as my original code seems logically sound.
assembly raspberry-pi arm
edited 2 days ago
cooperised
1,042712
asked 2 days ago
eddiewastaken
1768
2
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
3
Stack operations must be balanced. Judging by the code you've shown us, ifr0 == -1you'll skip thePOP {r0-r4}.
– Michael
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to saveR0when callinggetchar()? If not, you can just usepush {r1-r4}; bl getchar; pop {r1-r4};andr0is still the return status. Otherwise,push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4};Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.
– artless noise
2 days ago
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
My program creates a 2d array in memory with .skip 1000. It then populates that array with an input from stdin using this loop:
@@loop to store message in array
@outer for loop over rows
MOV r0,#0 @r0 = i (row index)
msgrowloop:
CMP r0,r2 @compare to nrows
BEQ msgendrowloop
@multiply/accumulate instruction
MLA r7, r3, r0, r6 @calculates the address of the first element in each row
@inner for loop over columns
MOV r1,#0 @r1 = j (column index)
msgcolumnloop:
CMP r1,r3 @compare to ncolumns
BEQ msgendcolumnloop
@@@store from stdin
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
@@@store from stdin end
@store r8 in memory and increase r7 by one byte
STRB r8,[r7],#1
ADD r1,r1,#1 @j += 1
B msgcolumnloop
msgendcolumnloop:
ADD r0,r0,#1 @i += 1
B msgrowloop
msgendrowloop:
@rest of the program...
Now, using this I get a segmentation error, but if I change my stdin function to this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0, #-1 @check if we are at end of file
MOV r8, r0 @move character to r8
POP {r0-r4}
BEQ msgendrowloop @exit loop when done
Instead of this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
It works perfectly. The logic here is confusing as my original code seems logically sound.
assembly raspberry-pi arm
edited 2 days ago
cooperised
1,042712
asked 2 days ago
eddiewastaken
1768
My program creates a 2d array in memory with .skip 1000. It then populates that array with an input from stdin using this loop:
@@loop to store message in array
@outer for loop over rows
MOV r0,#0 @r0 = i (row index)
msgrowloop:
CMP r0,r2 @compare to nrows
BEQ msgendrowloop
@multiply/accumulate instruction
MLA r7, r3, r0, r6 @calculates the address of the first element in each row
@inner for loop over columns
MOV r1,#0 @r1 = j (column index)
msgcolumnloop:
CMP r1,r3 @compare to ncolumns
BEQ msgendcolumnloop
@@@store from stdin
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
@@@store from stdin end
@store r8 in memory and increase r7 by one byte
STRB r8,[r7],#1
ADD r1,r1,#1 @j += 1
B msgcolumnloop
msgendcolumnloop:
ADD r0,r0,#1 @i += 1
B msgrowloop
msgendrowloop:
@rest of the program...
Now, using this I get a segmentation error, but if I change my stdin function to this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0, #-1 @check if we are at end of file
MOV r8, r0 @move character to r8
POP {r0-r4}
BEQ msgendrowloop @exit loop when done
Instead of this:
PUSH {r0-r4}
BL getchar @branch & link to getchar - reads single character from stdin
CMP r0,#-1 @check if we're at the end of file
BEQ msgendrowloop @if so, exit loop
MOV r8, r0 @move character to r8
POP {r0-r4}
It works perfectly. The logic here is confusing as my original code seems logically sound.
assembly raspberry-pi arm
assembly raspberry-pi arm
edited 2 days ago
cooperised
1,042712
asked 2 days ago
eddiewastaken
1768
edited 2 days ago
cooperised
1,042712
asked 2 days ago
eddiewastaken
1768
edited 2 days ago
cooperised
1,042712
edited 2 days ago
cooperised
1,042712
edited 2 days ago
cooperised
1,042712
1,042712
asked 2 days ago
eddiewastaken
1768
asked 2 days ago
eddiewastaken
1768
asked 2 days ago
eddiewastaken
1768
1768
2
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
3
Stack operations must be balanced. Judging by the code you've shown us, ifr0 == -1you'll skip thePOP {r0-r4}.
– Michael
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to saveR0when callinggetchar()? If not, you can just usepush {r1-r4}; bl getchar; pop {r1-r4};andr0is still the return status. Otherwise,push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4};Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.
– artless noise
2 days ago
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday
add a comment |
2
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
3
Stack operations must be balanced. Judging by the code you've shown us, ifr0 == -1you'll skip thePOP {r0-r4}.
– Michael
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to saveR0when callinggetchar()? If not, you can just usepush {r1-r4}; bl getchar; pop {r1-r4};andr0is still the return status. Otherwise,push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4};Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.
– artless noise
2 days ago
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday
2
2
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
3
3
Stack operations must be balanced. Judging by the code you've shown us, if
r0 == -1 you'll skip the POP {r0-r4}.– Michael
2 days ago
Stack operations must be balanced. Judging by the code you've shown us, if
r0 == -1 you'll skip the POP {r0-r4}.– Michael
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to save
R0 when calling getchar()? If not, you can just use push {r1-r4}; bl getchar; pop {r1-r4}; and r0 is still the return status. Otherwise, push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4}; Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.– artless noise
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to save
R0 when calling getchar()? If not, you can just use push {r1-r4}; bl getchar; pop {r1-r4}; and r0 is still the return status. Otherwise, push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4}; Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.– artless noise
2 days ago
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday
add a comment |
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2
Where does it segfault? Have you used a debugger to find out? The fact that it segfaulted means that your code was not logically sound, of course; but equally, the fact that it no longer segfaults with your modification does not mean that you have fixed it, just that it is no longer broken in the same way...
– cooperised
2 days ago
3
Stack operations must be balanced. Judging by the code you've shown us, if
r0 == -1you'll skip thePOP {r0-r4}.– Michael
2 days ago
You can also solve this by only using r0-r3 for intermediates (temporaries) that don't need to be saved over function calls. Do you really need to save
R0when callinggetchar()? If not, you can just usepush {r1-r4}; bl getchar; pop {r1-r4};andr0is still the return status. Otherwise,push {r0-r4}; bl getchar; cmp r0, #-1; pop {r0-r4};Using r4-r8 for value to be preserved over a function call as the EABI intends solves the issue without any code.– artless noise
2 days ago
Note also that the ARM ABI requires 8-byte stack alignment across function boundaries involving different translation units. If you can't guarantee 8-byte alignment then you'll get undefined behaviour. You can ensure that the stack remains 8-byte aligned by always pushing and popping even numbers of registers.
– cooperised
yesterday