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cassemblyarm
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Reputation: 2456

ARM - Load and Store assembly instructions

I am trying to load and store data from two different arm registers. 

int testing[64*1024]  __attribute__ ((aligned (8192)));
__asm__("MOV r0, %0" :: "r" (testing) : "r0");
__asm__("STR R5,[R0];");

In my initial attempt I tried to store some data pointed to by the register r0 to register r5. There are absolutely no compilation problems but the data in the register cannot be accessed.

It is the same case for Load as well.

LDR R1,[R0]



(gdb) info registers
r0             0xb6000  745472
r1             0x1      1
r2             0x0      0
r3             0xb6000  745472
r4             0x8961   35169
r5             0x0      0
r6             0x0      0
r7             0xbeba9664       3199899236
r8             0x0      0
r9             0xefb9   61369
r10            0xf02d   61485
r11            0x0      0
r12            0x0      0
sp             0xbeba9664       0xbeba9664
lr             0x89cb   35275
pc             0xeace   0xeace <test48+14>
cpsr           0x60000030       1610612784
(gdb) bt
#0  0x0000eace in test48 ()
#1  0x000089ca in main ()
(gdb) x/x $r5
0x0:    Cannot access memory at address 0x0
(gdb) x/x $r0
0xb6000 <testing>: 0x00000000

Essentially I am trying to achieve some memory inline addressing using ldr and str.

I took help of this guide while I was building my example

Any idea where I am going wrong

Upvotes: 0

Views: 3086

Answers (1)

Turbo J
Turbo J

Reputation: 7691

Your comment and your code do not match:

In my initial attempt I tried to store some data pointed to by the register r0 to register r5 [...] __asm__("STR R5,[R0];");

The instruction you wrote stores the value of R5 into the memory location that R0 points to. The register R5 does not point to any memory location - its value is 0x00 in your example.

The __asm__ statements do not declare the R5 register used in any way, so the compiler is free to put any temporary value or variable in it. This also explains:

(gdb) x/x $r5
0x0:    Cannot access memory at address 0x0

Your gdb command tries to access the memory location that R5 points to - but it does not point at any.

Upvotes: 3

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