From 6eca218f8eade55f7e7a15f3cc87bb814e6b7726 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Fri, 3 Apr 2020 19:43:10 +0000
Subject: [PATCH] Explain the booterr code a bit

---
 inc/booterr.asm | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/inc/booterr.asm b/inc/booterr.asm
index 8a8af3a..21dbcbc 100644
--- a/inc/booterr.asm
+++ b/inc/booterr.asm
@@ -1,18 +1,27 @@
-errcll:	pop dx
+errcll:	; put return address into dx
+	pop dx
 errjmp:	call dmpdx
 hlt:	hlt
 	jmp hlt
-dmpdx:	xor bx, bx
+dmpdx:	; setup bx and ah for int 10h call
+	xor bx, bx
 	mov ah, 0x0e
 	mov cl, 4
+	; this double-call is essentially a 4 times repeating loop
 	call dmpdx1
 dmpdx1:	call dmpdx2
-dmpdx2:	mov al, dh
+dmpdx2:	; grab highest nibble from dx
+	mov al, dh
+	; remove highest nibble from dx
 	shl dx, cl
+	; shift away second-highest nibble that we accidentally copied
 	shr al, cl
+	; map 0-9 to ascii codes for '0' to '9'
 	add al, 0x30
+	; if result is larger than '9', ...
 	cmp al, 0x3a
 	jl dmpdx3
+	; ... add 7 so we continue at 'A'
 	add al, 7
 dmpdx3:	int 0x10
 	ret