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crt0.s removed redundant instructions and added more asm comments

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Felix Jones 2019-09-17 23:25:51 +01:00
parent 898fbdadd0
commit 09a7c73004
1 changed files with 12 additions and 11 deletions
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23
crt0.s
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@ -51,12 +51,12 @@ __start:
@ set IRQ stack pointer
mov r0, #0x12
msr CPSR_cf, r0
msr CPSR_c, r0
ldr sp, =0x3007fa0
@ set user stack pointer
mov r0, #0x1f
msr CPSR_cf, r0
msr CPSR_c, r0
ldr sp, =0x3007f00
@ copy .data section to IWRAM
@ -89,19 +89,19 @@ MainIrqHandler:
stmdb sp!, {r0-r2,lr}
@ Disable all interrupts by writing to IME
mov r0, #0
strh r0, [r2, #8]
@ r2 (0x4000200) can be used as we only care about bit 0 being unset
strh r2, [r2, #8]
@ Acknowledge all received interrupts that were enabled in IE
ldr r3, [r2, #0]
and r0, r3, r3, lsr #16
strh r0, [r2, #2]
@ Switch to system mode
@ Switch from IRQ mode to system mode
@ cpsr_c = 0b000_10010u8 | 0b000_01101u8
mrs r2, cpsr
bic r2, r2, #0x1F
orr r2, r2, #0x1F
msr cpsr_cf, r2
orr r2, r2, #0xD
msr cpsr_c, r2
@ Jump to user specified IRQ handler
ldr r2, =__IRQ_HANDLER
@ -112,11 +112,12 @@ MainIrqHandler:
.Lreturn:
ldmia sp!, {lr}
@ Switch to IRQ mode
@ Switch from ??? mode to IRQ mode, disable IRQ
@ cpsr_c = ( !0b000_01101u8 & cpsr_c ) | 0b100_10010u8
mrs r2, cpsr
bic r2, r2, #0x1F
bic r2, r2, #0xD
orr r2, r2, #0x92
msr cpsr_cf, r2
msr cpsr_c, r2
@ Restore IRQ stack pointer and IME
ldmia sp!, {r0-r2,lr}