TQ2440 uboot---6.start.S in relocate part analysis, adr and ldr difference

There is such a paragraph in u-boot's start.S

relocate: /*relocate U-Boot to RAM */

       adr r0, _start /*r0 <- current position of code */

       ldr r1, _TEXT_BASE /* test if we run from flash or RAM */

       cmp r0, r1 /* don't reloc duringdebug */

       beq clear_bss

       ldr r2, _armboot_start

       ldr r3, _bss_start

       sub r2, r3, r2 /*r2 <- size of armboot */

       bl CopyCode2Ram /*r0: source, r1: dest, r2: size */

clear_bss:

       …….

Compare r0 and r1, if they are not equal, copy the code from flash to ram. But the question is how can we determine that r0 and r1 are not equal just from the two instructions adr and ldr?

Since the assembly can't handle it, then disassemble it. By assembling the above code, you can see that there are differences between the two instructions.

33d800b0 :

33d800b0: e24f00b8 sub r0, pc, #184; 0xb8

33d800b4: e51f107c ldr r1, [pc, #-124] ;33d80040 <_text_base>

33d800b8: e1500001 cmp r0, r1

33d800bc: 0a000003 beq 33d800d0

adr r0, _start ==> 33d800b0: e24f00b8 sub r0, pc, #184; 0xb8

Get the address of the label at pc-184 and assign it to r0. _start changes with the load address, so r0 changes. When running from nandflash, it actually runs at address 0x0 of sram, and the address of _start is 0x0; when running from 0x33d80000, the value of _start is 0x33d80000.

ldr r1, _TEXT_BASE ==>800b4: e51f107c ldr r1, [pc, #-124]

The literal meaning is to get the value at the address [pc, #-124], that is, to get the value at the address _TEXT_BASE. No matter how the link is made, the address of _TEXT_BASE will change, but the value TEXT_BASE at the address _TEXT_BASE will not change. This instruction assigns the value of _TEXT_BASE, that is, .word TEXT_BASE, to r1, that is, r1=TEXT_BASE=0x33d80000. This instruction has nothing to do with where the program is running. As long as TEXT_BASE is defined, then r1=TEXT_BASE.