From 8fd7e5e623647866db88b632347261e5d9f30da1 Mon Sep 17 00:00:00 2001
From: Silvan Jegen <s.jegen@gmail.com>
Date: Sat, 10 Aug 2019 17:40:59 +0200
Subject: Implement first version of ISR

---
 software/first/setup_timer_irq.S | 62 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

(limited to 'software/first/setup_timer_irq.S')

diff --git a/software/first/setup_timer_irq.S b/software/first/setup_timer_irq.S
index 4ae714f..97b5f6c 100644
--- a/software/first/setup_timer_irq.S
+++ b/software/first/setup_timer_irq.S
@@ -1,6 +1,7 @@
 .section .text
 .align 2
 .global setup_timer_irq
+.include "gpio.inc"
 .include "memory_map.inc"
 
 setup_timer_irq:
@@ -29,6 +30,65 @@ setup_timer_irq:
 	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:
-	mret
+	# 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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