bring bootasm and bootother more in sync

2bc72bdd · rsc · 5d1f4b8a · 2bc72bdd · 2bc72bdd
--- a/bootasm.S
+++ b/bootasm.S
@@ -9,10 +9,11 @@
 .set PROT_MODE_DSEG, 0x10        # kernel data segment selector
 .set CR0_PE_ON,      0x1         # protected mode enable flag
-.globl start                      # Entry point
+.globl start
 start:
-.code16                           # This runs in real mode
+  .code16                     # Assemble for 16-bit mode
  cli                         # Disable interrupts
+  cld                         # String operations increment
  # Set up the important data segment registers (DS, ES, SS).
  xorw    %ax,%ax             # Segment number zero
@@ -40,28 +41,21 @@ seta20.2:
  movb    $0xdf,%al               # 0xdf -> port 0x60
  outb    %al,$0x60
-# Switch from real to protected mode
+//PAGEBREAK!
-#  The descriptors in our GDT allow all physical memory to be accessed.
+  # Switch from real to protected mode, using a bootstrap GDT
-#  Furthermore, the descriptors have base addresses of 0, so that the
+  # and segment translation that makes virtual addresses 
-#  segment translation is a NOP, ie. virtual addresses are identical to
+  # identical to their physical addresses, so that the 
-#  their physical addresses.  With this setup, immediately after
+  # effective memory map does not change during the switch.
-#  enabling protected mode it will still appear to this code
+  lgdt    gdtdesc
-#  that it is running directly on physical memory with no translation.
+  movl    %cr0, %eax
-#  This initial NOP-translation setup is required by the processor
+  orl     $CR0_PE_ON, %eax
-#  to ensure that the transition to protected mode occurs smoothly.
+  movl    %eax, %cr0
-real_to_prot:
-  cli                         # Mandatory since we dont set up an IDT
+  # Jump to next instruction, but in 32-bit code segment.
-  lgdt    gdtdesc             # load GDT -- mandatory in protected mode
+  # Switches processor into 32-bit mode.
-  movl    %cr0, %eax          # turn on protected mode
-  orl     $CR0_PE_ON, %eax    #
-  movl    %eax, %cr0          #
-  ### CPU magic: jump to relocation, flush prefetch queue, and reload %cs
-  ### Has the effect of just jmp to the next instruction, but simultaneous
-  ### loads CS with $PROT_MODE_CSEG.
  ljmp    $PROT_MODE_CSEG, $protcseg
-#### we are in 32-bit protected mode (hence the .code32)
+  .code32                     # Assemble for 32-bit mode
-.code32
 protcseg:
  # Set up the protected-mode data segment registers
  movw    $PROT_MODE_DSEG, %ax    # Our data segment selector
@@ -71,13 +65,13 @@ protcseg:
  movw    %ax, %gs                # -> GS
  movw    %ax, %ss                # -> SS: Stack Segment
-  # Set up the stack pointer, growing downward from 0x7c00.
+  # Set up the stack pointer and call into C.
  movl    $start, %esp
+  call cmain
-  call cmain                      # finish the boot load from C.
+  # If cmain returns (it shouldn't), loop.
-                                  # cmain() should not return
 spin:
-  jmp spin                        # ..but in case it does, spin
+  jmp spin
 # Bootstrap GDT
 .p2align 2                                # force 4 byte alignment

--- a/bootother.S
+++ b/bootother.S
@@ -12,14 +12,19 @@
 # mp.c causes each non-boot CPU in turn to jump to start.
 # mp.c puts the correct %esp in start-4, and the place to jump
 # to in start-8.
+#
+# This code is identical to bootasm.S except:
+#   - it does not need to enable A20
+#   - it uses 0(start-4) for the %esp
+#   - it jumps to 0(start-8) instead of calling cmain
-.set PROT_MODE_CSEG,0x8         # code segment selector
+.set PROT_MODE_CSEG, 0x8         # kernel code segment selector
-.set PROT_MODE_DSEG,0x10        # data segment selector
+.set PROT_MODE_DSEG, 0x10        # kernel data segment selector
-.set CR0_PE_ON,0x1              # protected mode enable flag
+.set CR0_PE_ON,      0x1         # protected mode enable flag
 .globl start
 start:
-  .code16                     # This runs in real mode
+  .code16                     # Assemble for 16-bit mode
  cli                         # Disable interrupts
  cld                         # String operations increment
@@ -29,31 +34,21 @@ start:
  movw    %ax,%es             # -> Extra Segment
  movw    %ax,%ss             # -> Stack Segment
-  # Set up the stack pointer, growing downward from 0x7000-8.
+//PAGEBREAK!
-  movw    $start-8,%sp        # Stack Pointer
+  # Switch from real to protected mode, using a bootstrap GDT
+  # and segment translation that makes virtual addresses 
-  # Switch from real to protected mode
+  # identical to their physical addresses, so that the 
-  #  The descriptors in our GDT allow all physical memory to be accessed.
+  # effective memory map does not change during the switch.
-  #  Furthermore, the descriptors have base addresses of 0, so that the
+  lgdt    gdtdesc
-  #  segment translation is a NOP, ie. virtual addresses are identical to
+  movl    %cr0, %eax
-  #  their physical addresses.  With this setup, immediately after
+  orl     $CR0_PE_ON, %eax
-  #  enabling protected mode it will still appear to this code
+  movl    %eax, %cr0
-  #  that it is running directly on physical memory with no translation.
-  #  This initial NOP-translation setup is required by the processor
-  #  to ensure that the transition to protected mode occurs smoothly.
-  lgdt    gdtdesc             # load GDT -- mandatory in protected mode
-  movl    %cr0, %eax          # turn on protected mode
-  orl     $CR0_PE_ON, %eax    #
-  movl    %eax, %cr0          #
-  # CPU magic: jump to relocation, flush prefetch queue, and reload %cs
+  # Jump to next instruction, but in 32-bit code segment.
-  # Has the effect of just jmp to the next instruction, but simultaneous
+  # Switches processor into 32-bit mode.
-  # loads CS with $PROT_MODE_CSEG.
  ljmp    $PROT_MODE_CSEG, $protcseg
-# We are now in 32-bit protected mode (hence the .code32)
+  .code32                     # Assemble for 32-bit mode
-.code32
 protcseg:
  # Set up the protected-mode data segment registers
  movw    $PROT_MODE_DSEG, %ax    # Our data segment selector
@@ -63,10 +58,11 @@ protcseg:
  movw    %ax, %gs                # -> GS
  movw    %ax, %ss                # -> SS: Stack Segment
-  movl    start-8, %eax
  movl    start-4, %esp
+  movl    start-8, %eax
  jmp     *%eax
+# Bootstrap GDT
 .p2align 2                                # force 4 byte alignment
 gdt:
  SEG_NULLASM                             # null seg