irq_preempt.S

// Custom CSRs
#define meie0 0xbe0 // External interrupt enable
#define meip0 0xfe0 // External interrupt pending
#define mlei  0xfe4 // Lowest external interrupt, left shifted by 2.

_external_irq_vector:
	// Begin critical section
	addi sp, sp, -16
	sw a0, 0(sp)
	sw a1, 4(sp)

	// Save exception return state. No need to save mstatus because we know we
	// got here from M mode, and we know interrupts were enabled at the point
	// the interrupt fired.
	csrr a0, mepc
	sw a0, 8(sp)
	csrr a0, meie0
	sw a0, 12(sp)

	// Disable this and lower-priority interrupts. mlei returns the index of
	// the lowest-numbered pending & enabled external IRQ, left-shifted by 2.
	csrr a0, mlei
	la a1, irq_primask_table
	add a1, a1, a0
	lw a1, (a1)
	csrw meie0, a1

	// End of critical section. Enable preemption.
	csrsi mstatus, 0x8

	// Save caller saves. ISR may be an undecorated function.
	addi sp, sp, -64
	sw ra, 0 (sp)
	sw t0, 4 (sp)
	sw t1, 8 (sp)
	sw t2, 12(sp)
	sw a2, 16(sp)
	sw a3, 20(sp)
	sw a4, 24(sp)
	sw a5, 28(sp)
	sw a6, 32(sp)
	sw a7, 36(sp)
	sw t3, 40(sp)
	sw t4, 44(sp)
	sw t5, 48(sp)
	sw t6, 52(sp)

	// Let's go!
	la a1, irq_vector_table
	add a1, a1, a0
	jalr a1

	// Restore caller saves
	lw ra, 0 (sp)
	lw t0, 4 (sp)
	lw t1, 8 (sp)
	lw t2, 12(sp)
	lw a2, 16(sp)
	lw a3, 20(sp)
	lw a4, 24(sp)
	lw a5, 28(sp)
	lw a6, 32(sp)
	lw a7, 36(sp)
	lw t3, 40(sp)
	lw t4, 44(sp)
	lw t5, 48(sp)
	lw t6, 52(sp)
	addi sp, sp, 64

	// Begin critical section
	csrci mstatus, 0x8

	lw a0, 8(sp)
	csrw mepc, a0
	lw a0, 12(sp)
	csrw meie0, a0
	lw a0, 0(sp)
	lw a1, 4(sp)
	addi sp, sp, 16

	// Exception state restored, return to whence we came
	mret

.section .data, "aw"

// RAM-resident ISR function pointer table
.p2align 2
_external_irq_vector_table:
.rept 32
.word 0
.endr

// For each IRQ number, a mask where that bit and the bit corresponding to any
// IRQ of lower or equal priority is clear. The IRQs are grouped into
// relatively few priority levels (say 4 of them) to avoid blowing out the
// stack. When an IRQ's priority is adjusted, we disable IRQs and update the
// mask table.
_external_irq_primask_table:
.rept 32
.word 0
.endr