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.section .text
.align 2
.global setup_timer_irq
.include "gpio.inc"
.include "memory_map.inc"
setup_timer_irq:
addi sp, sp, -16 # Allocate stack frame
sw ra, 12(sp) # save return address to the stack
lw t0, irq_handler # load address of irq_handler
slli t0, t0, 1 # shift address left by one; LSB will be 0 which is direct mode (what we want)
csrw mtvec, t0 # write handler address and mode
# Here we cannot use csrsi because it can only set up to 5 bits
li t0, 0x80 # set 7th bit (indexed from 0)
csrw mie, t0 # activate machine timer interrupt
# write high value to the timercmp just to be sure we don't
# trigger an interrupt by mistake
li t0, 0x7FFFFFFF # high number
li t1, 0xFFFFFFFF # high number
li t2, MTIMECMP # load address of timecmp register
sw t0, 4(t2) # store the first number to the upper part first
sw t1, 0(t2) # then the second to the lower part
csrsi mstatus, 0x4 # set 3rd bit to activate machine interrupts.
lw ra, 12(sp) # load return address
addi sp, sp, 16 # deallocate stack frame
ret
# Unix C ABI has high context switch cost. The current RISC-V ABI
# requires (ignoring floating-point registers; from
# https://content.riscv.org/wp-content/uploads/2018/05/08.45-09.10-RISCV-20180509-FastInts.pdf
# ):
#
# addi sp, sp, -FRAMESIZE
# sw ra, x(sp) # 1 return address
# sw t0-t6, x(sp) # 7 temporaries
# sw a0-a7, x(sp) # 8 arguments
#
# jal c_handler
#
# lw a0-a7, x(sp) # 8 arguments
# lw t0-t6, x(sp) # 7 temporaries
# lw ra, x(sp) # 1 return address
# addi sp, sp, FRAMESIZE
# mret
irq_handler:
# save only the registers we use in this function
addi sp, sp, -24 # Allocate stack frame
sw ra, 20(sp) # save return address to the stack
sw t0, 16(sp) # save temporary
sw t1, 12(sp) # save temporary
sw t2, 8(sp) # save temporary
sw t3, 4(sp) # save temporary
li t0, GPIO_CTRL_ADDR # load base GPIO address
lw t1, GPIO_OUTPUT_VAL(t0) # load state
beqz t1, switchOnRed # if none is blinking yet, blink the red one
li t3, GPIO_RED_LED
beq t1, t3, switchOnBlue # if red is on, switch on blue LED
li t3, GPIO_BLUE_LED
beq t1, t3, switchOnGreen # if blue is on, switch on green LED
# here, neither the red or blue are on, so the one that is on
# must be the green one. In that case we just fall through to
# switch on the red LED
switchOnRed:
li t2, GPIO_RED_LED # load value indicating red LED
j exit
switchOnBlue:
li t2, GPIO_BLUE_LED # load value indicating blue LED
j exit
switchOnGreen:
li t2, GPIO_GREEN_LED # load value indicating green LED
j exit
exit:
sw t2, GPIO_OUTPUT_VAL(t0) # write new LED values to the right address
lw ra, 20(sp) # load return address
lw t0, 16(sp) # restore temporary
lw t1, 12(sp) # restore temporary
lw t2, 8(sp) # restore temporary
lw t3, 4(sp) # restore temporary
addi sp, sp, 24 # deallocate stack frame
mret # exit IRQ handler
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