From faad047ab22cbe989c208bff5ecb42608ecb8d7b Mon Sep 17 00:00:00 2001
From: Robert Morris
Date: Mon, 13 Sep 2010 15:34:44 -0400
Subject: change some comments, maybe more informative delete most comments
 from bootother.S (since copy of bootasm.S) ksegment() -> seginit() move more
 stuff from main() to mainc()

---
 bootasm.S | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

(limited to 'bootasm.S')

diff --git a/bootasm.S b/bootasm.S
index 56175ce..f5d1678 100644
--- a/bootasm.S
+++ b/bootasm.S
@@ -13,7 +13,7 @@
 .code16                       # Assemble for 16-bit mode
 .globl start
 start:
-  cli                         # Disable interrupts
+  cli                         # BIOS enabled interrupts ; disable
 
   # Set up the important data segment registers (DS, ES, SS).
   xorw    %ax,%ax             # Segment number zero
@@ -45,7 +45,8 @@ seta20.2:
   # Switch from real to protected mode, using a bootstrap GDT
   # and segment translation that makes virtual addresses 
   # identical to physical addresses, so that the 
-  # effective memory map does not change during the switch.
+  # effective memory map does not change after subsequent
+  # loads of segment registers.
   lgdt    gdtdesc
   movl    %cr0, %eax
   orl     $CR0_PE, %eax
@@ -57,7 +58,11 @@ seta20.2:
   # default to 32 bits after this jump.
   ljmp    $(SEG_KCODE<<3), $start32
 
-.code32                       # Assemble for 32-bit mode
+# tell the assembler to generate 0x66 prefixes for 16-bit
+# instructions like movw, and to generate 32-bit immediate
+# addresses.
+.code32
+
 start32:
   # Set up the protected-mode data segment registers
   movw    $(SEG_KDATA<<3), %ax    # Our data segment selector
-- 
cgit v1.2.3


From cf4b1ad90bcaeeb0c8458098c87948f61d408f94 Mon Sep 17 00:00:00 2001
From: Russ Cox
Date: Sat, 19 Feb 2011 21:17:55 -0500
Subject: xv6: formatting, cleanup, rev5 (take 2)

---
 bootasm.S | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)

(limited to 'bootasm.S')

diff --git a/bootasm.S b/bootasm.S
index f5d1678..3cc23e7 100644
--- a/bootasm.S
+++ b/bootasm.S
@@ -13,7 +13,7 @@
 .code16                       # Assemble for 16-bit mode
 .globl start
 start:
-  cli                         # BIOS enabled interrupts ; disable
+  cli                         # BIOS enabled interrupts; disable
 
   # Set up the important data segment registers (DS, ES, SS).
   xorw    %ax,%ax             # Segment number zero
@@ -21,10 +21,8 @@ start:
   movw    %ax,%es             # -> Extra Segment
   movw    %ax,%ss             # -> Stack Segment
 
-  # Enable A20:
-  #   For backwards compatibility with the earliest PCs, physical
-  #   address line 20 is tied low, so that addresses higher than
-  #   1MB wrap around to zero by default.  This code undoes this.
+  # Physical address line A20 is tied to zero so that the first PCs 
+  # with 2 MB would run software that assumed 1 MB.  Undo that.
 seta20.1:
   inb     $0x64,%al               # Wait for not busy
   testb   $0x2,%al
@@ -41,28 +39,21 @@ seta20.2:
   movb    $0xdf,%al               # 0xdf -> port 0x60
   outb    %al,$0x60
 
-//PAGEBREAK!
-  # Switch from real to protected mode, using a bootstrap GDT
-  # and segment translation that makes virtual addresses 
-  # identical to physical addresses, so that the 
-  # effective memory map does not change after subsequent
-  # loads of segment registers.
+  # Switch from real to protected mode.  Use a bootstrap GDT that makes
+  # virtual addresses map dierctly to  physical addresses so that the
+  # effective memory map doesn't change during the transition.
   lgdt    gdtdesc
   movl    %cr0, %eax
   orl     $CR0_PE, %eax
   movl    %eax, %cr0
-  
-  # This ljmp is how you load the CS (Code Segment) register.
-  # SEG_ASM produces segment descriptors with the 32-bit mode
-  # flag set (the D flag), so addresses and word operands will
-  # default to 32 bits after this jump.
-  ljmp    $(SEG_KCODE<<3), $start32
 
-# tell the assembler to generate 0x66 prefixes for 16-bit
-# instructions like movw, and to generate 32-bit immediate
-# addresses.
-.code32
+//PAGEBREAK!
+  # Complete transition to 32-bit protected mode by using long jmp
+  # to reload %cs and %eip.  The segment registers are set up with no
+  # translation, so that the mapping is still the identity mapping.
+  ljmp    $(SEG_KCODE<<3), $start32
 
+.code32  # Tell assembler to generate 32-bit code now.
 start32:
   # Set up the protected-mode data segment registers
   movw    $(SEG_KDATA<<3), %ax    # Our data segment selector
-- 
cgit v1.2.3