From a9f63b8cdb930d079cc69d492fe44dc8224c1e03 Mon Sep 17 00:00:00 2001
From: Christian Cunningham <cc@localhost>
Date: Wed, 16 Mar 2022 22:28:33 -0700
Subject: More comments

---
 src/exceptions/data.S      | 19 +++++++++++--------
 src/exceptions/fiq.S       |  7 ++++---
 src/exceptions/irq.S       |  1 +
 src/exceptions/prefetch.S  |  8 ++++----
 src/exceptions/svc.S       | 12 +++++++++---
 src/exceptions/undefined.S | 16 ++++++----------
 6 files changed, 35 insertions(+), 28 deletions(-)

(limited to 'src/exceptions')

diff --git a/src/exceptions/data.S b/src/exceptions/data.S
index 656ba96..e3add71 100644
--- a/src/exceptions/data.S
+++ b/src/exceptions/data.S
@@ -9,21 +9,24 @@ data:
 	ldr r2, =data_msg
 	bl draw_string
 	// Output return address
-	mov r0, #13
-	mov r1, #15
+	mov r0, #80
+	mov r1, #0
 	mov r2, r4
 	sub r2, #8
 	bl draw_hex32
-	mov r0, #22
-	mov r1, #15
+	// Output the data at the address
+	mov r0, #80
+	mov r1, #1
 	ldr r2, [r4, #-8]
 	bl draw_hex32
-	mov r0, #32
-	mov r1, #15
+	// Output the Program Status
+	mov r0, #80
+	mov r1, #2
 	mrs r2, spsr
 	bl draw_hex32
-	mov r0, #41
-	mov r1, #15
+	// Output the data-fault register
+	mov r0, #80
+	mov r1, #3
 	mrc p15, 0, r2, c5, c0, 0 //// https://developer.arm.com/documentation/ddi0464/d/System-Control/Register-descriptions/Data-Fault-Status-Register?lang=en
 	bl draw_hex32
 	ldmfd sp!, {r0-r12,lr}
diff --git a/src/exceptions/fiq.S b/src/exceptions/fiq.S
index f323a02..005ed76 100644
--- a/src/exceptions/fiq.S
+++ b/src/exceptions/fiq.S
@@ -6,13 +6,17 @@ fiq:
 	bl c_fiq_handler
 	cmp r0, #1
 	bne 1f
+	// Schedule if interrupted a thread
 	mrs r1, spsr
 	and r1, r1, #0x1f
 	cmp r1, #0x10
 	bne 1f
 	ldmfd sp!, {r0-r12,lr}
+	// Don't skip missed instruction upon return
 	sub lr, #4
 	push {r3}
+	// Store the instruction in a special area for
+	//  future processing
 	ldr r3, =irqlr
 	str lr, [r3, #0]
 	pop {r3}
@@ -21,6 +25,3 @@ fiq:
 1:
 	ldmfd sp!, {r0-r12,lr}
 	subs pc, lr, #4
-
-.section .data
-fiq_msg:	.asciz "FIQ\n"
diff --git a/src/exceptions/irq.S b/src/exceptions/irq.S
index 025fc41..db8b4cd 100644
--- a/src/exceptions/irq.S
+++ b/src/exceptions/irq.S
@@ -3,6 +3,7 @@
 irq:
 	cpsid aif
 	push {r0,r1,r2,r3,r4,r5,r6,r7,r8,r9,r10,r11,r12,lr}
+	// Run IRQ handler
 	bl c_irq_handler
 	pop  {r0,r1,r2,r3,r4,r5,r6,r7,r8,r9,r10,r11,r12,lr}
 	subs pc, lr, #4
diff --git a/src/exceptions/prefetch.S b/src/exceptions/prefetch.S
index 1aeba10..5166d00 100644
--- a/src/exceptions/prefetch.S
+++ b/src/exceptions/prefetch.S
@@ -4,13 +4,13 @@ prefetch:
 	cpsid aif
 	stmfd sp!, {r0-r12,lr}
 	mov r4, lr
-	mov r0, #0
-	mov r1, #16
+	mov r0, #98
+	mov r1, #0
 	ldr r2, =prefetch_msg
 	bl draw_string
 	// Output return address
-	mov r0, #17
-	mov r1, #16
+	mov r0, #98
+	mov r1, #1
 	mov r2, r4
 	bl draw_hex32
 	ldmfd sp!, {r0-r12,lr}
diff --git a/src/exceptions/svc.S b/src/exceptions/svc.S
index 3fb534e..7c8a9cc 100644
--- a/src/exceptions/svc.S
+++ b/src/exceptions/svc.S
@@ -3,8 +3,10 @@
 svc:
 	cpsid aif
 	stmfd sp!, {r0-r12,lr}
+	// Get the SVC Exception #
 	ldr r0, [lr, #-4]
 	bic r0, #0xFF000000
+	// Jump to the appropriate Call
 	cmp r0, #5
 	bgt svc_exit
 	beq svc_000005
@@ -18,11 +20,11 @@ svc:
 	beq svc_000001
 	cmp r0, #0
 	beq svc_000000
-svc_000000:
+svc_000000: // SYS_YIELD
 	bl yield
 	ldmfd sp!, {r0-r12,lr}
 	b schedule
-svc_000001: // Get time
+svc_000001: // SYS_TIME
 	mov r2, #0x3004
 	movt r2, #0x3F00
 	ldr r0, [r2, #4] // <- SYS_TIMER_CLO
@@ -50,13 +52,15 @@ svc_000004: // Lock Lock (usr_r0 = struct Lock*)
 1:	clrex
 	ldrex r2, [r0, #0]
 	cmp r2, #0
+	// If it is not available, wait-queue the thread
 	bne svc_000004_delay_mutex
+	// Otherwise lock it
 	strexeq r2, r1, [r0, #0]
 	teq r2, #0
 	bne 1b
 	dmb
 	b svc_exit
-svc_000004_delay_mutex:
+svc_000004_delay_mutex: // Wait-queue the current thread
 	// r0 = struct Lock* m
 	bl sched_mutex_yield
 	ldmfd sp!, {r0-r12,lr}
@@ -65,9 +69,11 @@ svc_000005: // Release Lock
 	ldr r0, [sp, #0] // struct Lock* m
 	mov r1, #0
 	dmb
+	// Unlock
 	str r1, [r0, #0]
 	dsb
 	sev
+	// Awake any threads waiting for this lock
 	bl sched_mutex_resurrect
 	ldmfd sp!, {r0-r12,lr}
 	b schedule
diff --git a/src/exceptions/undefined.S b/src/exceptions/undefined.S
index ef00735..2d4b90a 100644
--- a/src/exceptions/undefined.S
+++ b/src/exceptions/undefined.S
@@ -4,23 +4,19 @@ undefined:
 	cpsid aif
 	stmfd sp!, {r0-r12,lr}
 	ldr r4, [lr, #-4]
-	mov r0, #0
-	mov r1, #17
+	mov r0, #62
+	mov r1, #0
 	ldr r2, =undefined_msg
 	bl draw_string
-	mov r0, #18
-	mov r1, #17
+	mov r0, #62
+	mov r1, #1
 	mov r2, r4
 	bl draw_hex32
-	mov r0, #27
-	mov r1, #17
-	ldr r2, =undefined_at
-	bl draw_string
 	// Output lr
 	ldr r0, [sp, #0x34]
 	sub r2, r0, #4
-	mov r0, #31
-	mov r1, #17
+	mov r0, #62
+	mov r1, #2
 	bl draw_hex32
 	// Skip instruction for now
 	// In future, 
-- 
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