Good day,
I am trying to figure how does U-Boot relocates itself on Cyclone Edition board (DK-1C20). Probably this should be an easy question for most developers familiar with NIOS, but I can't find the answer since I am new to this platform.
So, this is my question: ===================
The following lines are located in the early beginning of start.s:
mov %g7, %o7 /* %o7 has return addr -- save in %g7 use later */
...
pfx %hi(__u_boot_cmd_end) movi %g5, %lo(__u_boot_cmd_end) pfx %xhi(__u_boot_cmd_end) movhi %g5, %xlo(__u_boot_cmd_end) /* %g5 <- end address */
lsli %g7, 1 /* mem = retaddr << 1 */ mov %g6, %g7 subi %g6, 4 /* %g6 <- src addr */ ld %g7, [%g7] /* %g7 <- dst addr */
1: cmp %g7, %g5 skps cc_nz br 2f nop /* delay slot */
ld %g0, [%g6] addi %g6, 4 /* src++ */ st [%g7], %g0 addi %g7, 4 /* dst++ */ br 1b nop /* delay slot */
I don't understand this particular line of code:
ld %g7, [%g7]
Per my understanding, %g7 keeps the return address. This address should point to the next instruction of code just after the delay slot of CALL or BSR used to call u-boot. So, if %g7 keeps this address, than [%g7] keeps the code (instruction bytes) located by this address. In this case "ld %g7, [%g7]" loads 4 bytes from code segment to the %g7.
Later this value (instruction bytes) is compared with %g5 that keeps the end of data segment according to u-boot.lds. I can't understand why code address is being compared with instruction bytes. Please explain if possible - what does [%g7] keep and where am I wrong?
Thank you!