more comments in entryother.S

3431cd49 · Robert Morris · 0a69dc9b · 3431cd49 · 3431cd49
--- a/entryother.S
+++ b/entryother.S
@@ -17,37 +17,43 @@
 # place to jump to (mpenter) in start-8, and the physical address
 # of entrypgdir in start-12.
 #
-# This code is identical to bootasm.S except:
-#   - it does not need to enable A20
-#   - it uses the address at start-4, start-8, and start-12
+# This code combines elements of bootasm.S and entry.S.

 .code16           
 .globl start
 start:
  cli            

+  # Zero data segment registers DS, ES, and SS.
  xorw    %ax,%ax
  movw    %ax,%ds
  movw    %ax,%es
  movw    %ax,%ss

+  # Switch from real to protected mode.  Use a bootstrap GDT that makes
+  # virtual addresses map directly 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

 //PAGEBREAK!
+  # Complete the transition to 32-bit protected mode by using a long jmp
+  # to reload %cs and %eip.  The segment descriptors are set up with no
+  # translation, so that the mapping is still the identity mapping.
  ljmpl    $(SEG_KCODE<<3), $(start32)

-.code32
+.code32  # Tell assembler to generate 32-bit code now.
 start32:
-  movw    $(SEG_KDATA<<3), %ax
-  movw    %ax, %ds
-  movw    %ax, %es
-  movw    %ax, %ss
-  movw    $0, %ax
-  movw    %ax, %fs
-  movw    %ax, %gs
+  # Set up the protected-mode data segment registers
+  movw    $(SEG_KDATA<<3), %ax    # Our data segment selector
+  movw    %ax, %ds                # -> DS: Data Segment
+  movw    %ax, %es                # -> ES: Extra Segment
+  movw    %ax, %ss                # -> SS: Stack Segment
+  movw    $0, %ax                 # Zero segments not ready for use
+  movw    %ax, %fs                # -> FS
+  movw    %ax, %gs                # -> GS

  # Turn on page size extension for 4Mbyte pages
  movl    %cr4, %eax

--- a/main.c
+++ b/main.c
@@ -98,10 +98,11 @@ startothers(void)
  }
 }

-// Boot page table used in entry.S and entryother.S.
-// Page directories (and page tables), must start on a page boundary,
-// hence the "__aligned__" attribute.  
-// Use PTE_PS in page directory entry to enable 4Mbyte pages.
+// The boot page table used in entry.S and entryother.S.
+// Page directories (and page tables) must start on page boundaries,
+// hence the __aligned__ attribute.  
+// PTE_PS in a page directory entry enables 4Mbyte pages.
+
 __attribute__((__aligned__(PGSIZE)))
 pde_t entrypgdir[NPDENTRIES] = {
  // Map VA's [0, 4MB) to PA's [0, 4MB)