An Intro to Kernel Development - MMU - Part 7

In the previous blog we parsed the DTB to find the actual total physical RAM. now we have to pass the parsed value from the dtb parsing helper to kmain, Initially i tried to call this helper after jumping into kmain, then i ran to Page fault for the DTB memory location, instead of add the page table entry for that location, which would have be the easier solution. I tried to write it to a Location that is already mapped and hoped that location is never writtn before we reach the kmain, but for some reason the address got overwritten, then instead of fix that, i tried to put on the stack, this also didnt work, as we have MMU init, which is ducking with the stack, so finally i found a register that seemed untouched and i have used that and it worked perfectly, this is still a hack, in future if i add something here, i need to be sure that, i wont write anything to r13 register ie, after the get_ram call and before kmain call. then i got into rabbit hole beleiving that value is read correctly in the kmain, and was trying to debug that for a long time, then i finally realized that the value is read correctly. the actual issue was disguised by two things, - my uart_printf doesnt support print u64 values - i have assigned a block of memory in linker script from my bitmap array, but i inited it in my C with extern like "extern uint8_t *__block_memory_start" i was thinking like, if i can take the starting address, i do the indexing maually with pointer arthimtic, but turns out, with extern, it will take try to take an pointer value from the block, so it will always return 0 "extern uint8_t __block_memory_start[]" just by declaring it as an array, i was able to fix it i have streamed the debugging session where i go through the mental breakdown below think about these two bugs.