On Fri, 03 Jan 2003 11:47:22 +0800, Laudney Ren bren@sjtu.edu.cn writes:
I'm reading cpu/mpc8xx/start.S and found the following:
/* * Calculate absolute address in FLASH and jump there */ lis r3, CFG_MONITOR_BASE@h ori r3, r3, CFG_MONITOR_BASE@l addi r3, r3, in_flash - _start + EXC_OFF_SYS_RESET mtlr r3 blrWhy do we need to calculate the absolutely address? Why should we do it here, instead of everywhere?
U-Boot (aka ppcboot - since you are looking at cpu/mpc8xx) is compiled using the "-mrelocatable" flag i.e. my understanding of this is that all text references will be relative so that the code may be relocated at run time.
The bit of code above is special - at this point the CPU will be executing at whatever address the reset configuration dictates (for 8xx I think the CPU will read from the data bus at hard reset, which the board designer will have wired up for you - I forget what the options are, but just assume the address will be somewhere that you don't want it to be :-). In addition, the memory controller will be set up so that the boot device (e.g. flash) appears repeated throughout the entire physical address space (see the hard reset state of OR0/BR0).
CFG_MONITOR_BASE is the physical address where you want the boot device to reside. It will be used to program the memory controller registers which select the boot device (OR0/BR0). As soon as the mask in OR0 is programmed correctly, the boot device will disappear from the entire address space, and be accessible only at the base address programmed in BR0.
This code jumps to the address that the boot device will appear at after you program the memory controller registers, so that the boot device will not disappear out from underneath you.
Because the code is relocatable you can jump to any address you like - as long as the offset within the boot device is correct. "_start" is the address where the CPU will "start" execution - i.e. the hard reset vector. "in_flash" is where we want to continue execution after the jump (its the next instruction after the "blr" above - therefore this really isn't a jump because you simply execute the next instruction). EXC_OFF_SYS_RESET is added because we need the address to be relative to the start of the boot device, not the hard reset vector.
I think I got in over my head here - I started out trying to keep this answer simple, but there is too much background you need to know to understand this code. The 8xx (and 8260) are very configurable - there are other ways you could do this, this is just the way ppcboot decided to do it. This code will work regardless of the hardware reset configuration - so I quite like it.
I hope I've helped rather than hindered :-) In particular, I hope I've got all the above correct - I'm sure someone will correct me if I'm wrong. Cheers! Murray... -- Murray Jensen, CSIRO Manufacturing & Infra. Tech. Phone: +61 3 9662 7763 Locked Bag No. 9, Preston, Vic, 3072, Australia. Fax: +61 3 9662 7853 Internet: Murray.Jensen@csiro.au
Hymod project: http://www.msa.cmst.csiro.au/projects/Hymod/
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