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.drasko...@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 */ > > > 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 > > _______________________________________________ > U-Boot mailing list > U-Boot@lists.denx.de > http://lists.denx.de/mailman/listinfo/u-boot > _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
