From 0402071d7af9ccaa55bd333ada281ca584347120 Mon Sep 17 00:00:00 2001
From: Alexander Kuleshov <kuleshovmail@gmail.com>
Date: Fri, 11 Dec 2015 01:23:35 +0600
Subject: [PATCH] boot0 refactoring

The patch provides refactoring for the sys/boot/pc32/boot0.S
assembly file to imrove readability and comprehensibility of
assembly code.

The main changes are:

Two new symbols added: the _SECTOR_FIELD_OFF which provides
offset for the 'sector' field in CHS and the _DATA_OFF which
provides offset for the boot0 data from the partition table.
We can use these symbols instead of magic numbers in the code.

The second change is updating comments in some place.

The patch was tested as in virtual machine as in real hardware.

---
 sys/boot/pc32/boot0/boot0.S | 33 ++++++++++++++++++++-------------
 sys/boot/pc32/bootasm.h     |  2 +-
 2 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/sys/boot/pc32/boot0/boot0.S b/sys/boot/pc32/boot0/boot0.S
index 9f85b47..553da17 100644
--- a/sys/boot/pc32/boot0/boot0.S
+++ b/sys/boot/pc32/boot0/boot0.S
@@ -91,6 +91,11 @@
 		.set _FAKE,0x0			# Fake partition entry
 		.set _MNUOPT,0xc		# Menu options
 
+		.set _SECTOR_FIELD_OFF,-0xe	# Offset to Sector field
+						# in the fake partition
+		.set _DATA_OFF,PRT_OFF+_SECTOR_FIELD_OFF # Offset to boot0 variables
+							 # from partition table
+
 		.globl start			# Entry point
 		.code16				# This runs in real mode
 
@@ -107,7 +112,8 @@ start:		cld				# String ops inc
 		movw $MEM_BIOS_LADDR,%sp	#  stack
 	
 		/*
-		 * Copy this code to the address it was linked for
+		 * Copy this code to the address it was linked for.
+		 * Base address after relocation is - 0x600.
 		 */
 		movw %sp,%si			# Source
 		movw $start,%di			# Destination
@@ -126,11 +132,11 @@ start:		cld				# String ops inc
 		stosw				#  them
 
 		/*
-		 * Relocate to the new copy of the code.  Do not make
-		 * assumptions with regard to a relative-PC near jump
-		 * capability.
+		 * Set C:H:S to 0:0:1 and relocate to the new copy of
+		 * the code.  Do not make assumptions with regard to
+		 * a relative-PC near jump capability.
 		 */
-		incb -0xe(%di)			# Sector number
+		incb _SECTOR_FIELD_OFF(%di)	# Sector number
 		pushw $main			# Jump to relocated code
 		retw
 
@@ -171,12 +177,13 @@ main.2: 	movb %dl,_FAKE(%bp)		# Save drive number
 		 * that the table starts 2 bytes earlier than you would
 		 * expect as the bootable flag is after it in the block)
 		 */
-		movw $(partbl+0x4),%bx		# Partition table (+4)
-		xorw %dx,%dx			# Item number
+		movw $(partbl+0x4),%bx		# Partition table (+4 - type)
+		xorw %dx,%dx			# Partition entry item number
 
 		/*
 		 * Loop around on the partition table, printing values until
-		 * we pass a 256 byte boundary. The end of loop test is at
+		 * we pass a 256 (4x 64 byte partitions) byte boundary. The
+		 * end of loop test is at
 		 * main.5.
 		 */
 main.3: 	movb %ch,-0x4(%bx)		# Zero active flag (ch == 0)
@@ -375,8 +382,8 @@ main.13:	pushw %bx			# Save
 		movw $start,%bx			# Data to write
 		movb $0x3,%ah			# Write sector
 		callw intx13			#  to disk
-main.14:	popw %si			# Restore
-		popf				# Restore
+main.14:	popw %si			# Restore partition
+		popf				# Restore eflags
 
 		/*
 		 * If going to next drive, replace drive with selected one.
@@ -448,10 +455,10 @@ putchr:
 putchr: 	
 		movb $0x01,%ah			# BIOS: Send
 bioscom:
-		pusha				# Save
+		pusha				# Save (preserve driver number)
 		xorw %dx,%dx			# Use COM1
 		int $0x14			#  Character
-		popa				# Restore
+		popa				# Restore (restore driver number)
 		retw				# To caller
 #endif
 		
@@ -540,7 +547,7 @@ os_linux:	.ascii "Linu"; .byte 'x'|0x80
 os_dfbsd:	.ascii "DF/F"
 os_bsd: 	.ascii "BS";   .byte 'D'|0x80
 
-		.org PRT_OFF-0xe,0x90
+		.org _DATA_OFF,0x90
 
 		.word B0MAGIC			# Magic number
 
diff --git a/sys/boot/pc32/bootasm.h b/sys/boot/pc32/bootasm.h
index c538271..11fb018 100644
--- a/sys/boot/pc32/bootasm.h
+++ b/sys/boot/pc32/bootasm.h
@@ -55,7 +55,7 @@
 #define BDA_SCR		0x449		/* Video mode		*/
 #define BDA_POS		0x450		/* Cursor position	*/
 #define BDA_BOOT	0x472		/* Boot howto flag	*/
-#define BDA_NHRDRV	0x475
+#define BDA_NHRDRV	0x475		/* Number of drivers found by BIOS */
 #define BDA_KEYBOARD	0x496		/* BDA byte with keyboard bit */
 
 /*
-- 
2.4.6