I am trying to get the the address of a label in thumb assembly and I am having some trouble.
I already read this post but that cannot help me and I will explain why.
I am writing an simple program with Thumb assembly ( unfortunately I cannot use Thumb2 ).
Let's consider this code:
.arch armv5te .syntax unified .text .thumb .thumb_func thumbnow: 0x0 PUSH {LR} 0x2 LDR R0, =loadValues 0x4 POP {PC} .align loadValues: 0x8 .word 0xdeadbee1 0xC .word 0xdeadbee2 0x10 .word 0xdeadbee3
I am using the arm-linux-gnueabi toolchain to assemble that.
My microcontroller doesn't have an MMU so the memory address are static, no virtual pages etc.
The thing that I am trying to do is to make R0 having the value of 0x8 here so that then I can access the three words like this:
LDR R1, [R0] LDR R2, [R0,#4] LDR R3, [R0,#8]
This is not possible with LDR though because the value in the word is not possible to fit in a MOV command. The documentation of the assembler states that if the value cannot fit in a MOV command then it will put the value in a literal pool.
So my question is, is it possible in Thumb assembly to get the actual address of the label if the content of the address cannot fit in a MOV command?
Starting with this
.thumb
ldr r0,=hello
adr r0,hello
nop
nop
nop
nop
hello:
.word 0,1,2,3
gives this unlinked
00000000 <hello-0xc>:
0: 4806 ldr r0, [pc, #24] ; (1c <hello+0x10>)
2: a002 add r0, pc, #8 ; (adr r0, c <hello>)
4: 46c0 nop ; (mov r8, r8)
6: 46c0 nop ; (mov r8, r8)
8: 46c0 nop ; (mov r8, r8)
a: 46c0 nop ; (mov r8, r8)
0000000c <hello>:
c: 00000000 andeq r0, r0, r0
10: 00000001 andeq r0, r0, r1
14: 00000002 andeq r0, r0, r2
18: 00000003 andeq r0, r0, r3
1c: 0000000c andeq r0, r0, r12
linked
00001000 <hello-0xc>:
1000: 4806 ldr r0, [pc, #24] ; (101c <hello+0x10>)
1002: a002 add r0, pc, #8 ; (adr r0, 100c <hello>)
1004: 46c0 nop ; (mov r8, r8)
1006: 46c0 nop ; (mov r8, r8)
1008: 46c0 nop ; (mov r8, r8)
100a: 46c0 nop ; (mov r8, r8)
0000100c <hello>:
100c: 00000000 andeq r0, r0, r0
1010: 00000001 andeq r0, r0, r1
1014: 00000002 andeq r0, r0, r2
1018: 00000003 andeq r0, r0, r3
101c: 0000100c andeq r1, r0, r12
both ways r0 will return the address to the start of data from which you can then offset into that data from the caller or wherever.
.thumb
adr r0,hello
nop
nop
nop
arm-none-eabi-as so.s -o so.o
so.s: Assembler messages:
so.s:2: Error: address calculation needs a strongly defined nearby symbol
So the tool won't turn that into a load from the pool for you.
For what you want to do I think the pc relative add (adr) is the best you are going to get. You can try other toolchains as all of this is language and toolchain specific (assembly language is defined by the assembler not the target and for each toolchain (with an assembler) there can be differences in the language). Over time within gnu, how the linker and assembler worked together has changed, the linker patches up things it didn't used to.
You could of course go into the linker and add code to it to perform this optimization, the problem is most likely that by link time the linker is looking to resolve an address in the pool which is easy for it to do it doesn't have to change the instruction, the assembler would have to leave information for the linker that this is not just a fill this memory location with an address thing, either you modify gas to allow adr to work, and then if the linker cant resolve it within the instruction then the linker bails out with an error.'
Or you could just hard-code what you want and maintain it. I am not sure why the adr solution isn't adequate.
mov r0,#8 is a valid thumb instruction.