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explain some of the .option directives in _enter()

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gil 2024-05-15 23:33:36 -05:00
parent e409015ff8
commit 7d658ad704
1 changed files with 9 additions and 4 deletions
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13
src/main.rs
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@ -15,11 +15,16 @@ unsafe extern "C" fn _enter() -> ! {
use core::arch::asm;
asm!(
// before we use the `la` pseudo-instruction for the first time,
// we need to set `gp` (google linker relaxation)
".option push",
".option norelax",
// we need to set `gp` (google linker relaxation)
".option push", // pushes the current option stack
".option norelax", // disable relaxation so we can properly set `gp`
// this instruction compiles to:
// 1:
// auipc gp, %pcrel_hi(_global_pointer)
// addi gp, gp, %pcrel_lo(1b)
// if relaxation were on, this would simply be `mv gp, gp` or `addi gp, gp, 0`
"la gp, _global_pointer",
".option pop",
".option pop", // pops the current option stack
// set the stack pointer
"la sp, _init_stack_top",