4 files changed, 72 insertions, 26 deletions
diff --git a/software/first/delay.S b/software/first/delay.S
index 4df998a..656625f 100644
--- a/software/first/delay.S
+++ b/software/first/delay.S
@@ -16,10 +16,8 @@ delay:
 	li t2, MTIME_FREQUENCY # get clock freq (approx.)
 	mul t2, t2, a0         # multiply milliseconds with freq
 	add t2, t1, t2         # target mtime is now in t2
-
-1:
-	lw t1, 0(t0)      # load value of the timer
-	blt t1, t2, 1b    # keep looping until target mtime has been reached
+	li t3, MTIMECMP        # load address of MTIMECMP register
+	sw t2, 0(t3)           # store target time to MTIMECMP register. This only stores 32 bits so I am not sure if that is correct...
 
 	lw ra, 12(sp)   # load return address
 	addi sp, sp, 16 # deallocate stack frame
diff --git a/software/first/first.c b/software/first/first.c
index 7505d6c..889e05b 100644
--- a/software/first/first.c
+++ b/software/first/first.c
@@ -5,28 +5,16 @@
 #include "super_blink.h"
 
 int main() {
-		int error = 0;
-		int ledNum = 0;
-		int colours[NUM_LEDS] = {
-			GREEN_LED,
-			RED_LED,
-			BLUE_LED,
-		};
-
 		setup_GPIO();
 		setup_timer_irq();
 
-		while (!error) {
-			set_LED(colours[ledNum], ON);
-			delay(DELAY);
-
-			error = set_LED(colours[ledNum], OFF);
-			delay(DELAY);
-
-			ledNum++;
+		// set a delay so the first interrupt handler can fire
+		delay(DELAY);
 
-			if (ledNum >= NUM_LEDS) {
-				ledNum = 0;
-			}
+		while (1) {
+			// the idea being that delay sets the timer
+			// for the first interrupt. In the interrupt
+			// routine, we set the next delay so all we need
+			// to do here is to loop endlessly
 		}
 }
diff --git a/software/first/gpio.inc b/software/first/gpio.inc
index 37caeba..9617fdb 100644
--- a/software/first/gpio.inc
+++ b/software/first/gpio.inc
@@ -4,5 +4,5 @@
 
 .equ  GPIO_RGB_PINS,  0x680000
 .equ  GPIO_RED_LED,   0x400000
-.equ  GPIO_BLUE_LED,   0x200000
-.equ  GPIO_GREEN_LED,  0x080000
+.equ  GPIO_BLUE_LED,  0x200000
+.equ  GPIO_GREEN_LED, 0x080000
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