[U-Boot] Relocation confusion
Hi all, I have some questions regarding start.S code for arm926ejs. Looking at the relocation code of u-boot:
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 during debug */ beq stack_setup
ldr r2, _armboot_start ldr r3, _bss_start sub r2, r3, r2 /* r2 <- size of armboot */ add r2, r0, r2 /* r2 <- source end address */
1) What is exactly difference between _start and _armboot_start ? I can see in the same file definition of _armboot_start :
.globl _armboot_start _armboot_start: .word _start
So, I am confused there - it looks like they are the same thing.
2) Line: sub r2, r3, r2 is logical, but after we do : add r2, r0, r2 /* r2 <- source end address */ to find source end address. But if _start and _armboot_start are the same, we already have this source end address : it is _bss_start.
3) Based on this, if _armboot_start and _start are not the same thing, I could guess that _start is pointing at 0x0 of the flash (or SDRAM), i.e. it is uP entry point after reset, and _armboot_start is the place where we start to put u-boot instructions (TEXT section). That would mean that we either have a) some program uP will execute before u-boot. What could that ever be? b) some section of u-boot that goes before TEXT section, and will never be relocated to RAM _TEXT_BASE. What could that ever be?
Thanks for your answers.
Best regards, Drasko DRASKOVIC
Hi Drasko DRASKOVIC,
"adr" is to calculate a runtime address by PC+offset. "adr r0, _start" is like: "add r0, pc, #0x80" for example.
e.g. TEXT_BASE == 0x1000 0000 and you are using u-boot to boot from flash.
link address of _start probably equals to 0x1000 0000 the location _armboot_start stores the link address of _start
Therefore, adr r0, _start you probably got r0 == 0.
ldr r2, _armboot_start you probably got r2 == 0x1000 0000.
It is just some assembly tricks.
regards
Po-Yu Chuang
2009/4/22 Drasko DRASKOVIC drasko.draskovic@gmail.com
Hi all, I have some questions regarding start.S code for arm926ejs. Looking at the relocation code of u-boot:
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 during debug */ beq stack_setup
ldr r2, _armboot_start ldr r3, _bss_start sub r2, r3, r2 /* r2 <- size of armboot */ add r2, r0, r2 /* r2 <- source end address */
- What is exactly difference between _start and _armboot_start ? I can see
in the same file definition of _armboot_start :
.globl _armboot_start _armboot_start: .word _start
So, I am confused there - it looks like they are the same thing.
- Line:
sub r2, r3, r2 is logical, but after we do : add r2, r0, r2 /* r2 <- source end address */ to find source end address. But if _start and _armboot_start are the same, we already have this source end address : it is _bss_start.
- Based on this, if _armboot_start and _start are not the same thing, I
could guess that _start is pointing at 0x0 of the flash (or SDRAM), i.e. it is uP entry point after reset, and _armboot_start is the place where we start to put u-boot instructions (TEXT section). That would mean that we either have a) some program uP will execute before u-boot. What could that ever be? b) some section of u-boot that goes before TEXT section, and will never be relocated to RAM _TEXT_BASE. What could that ever be?
Thanks for your answers.
Best regards, Drasko DRASKOVIC
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Po-Yu Chuang