From 70bb19100eaa979c58a5513f29fec1b4a00634f6 Mon Sep 17 00:00:00 2001
From: Kartik Agaram <vc@akkartik.com>
Date: Sat, 19 Dec 2020 21:22:28 -0800
Subject: 7357

---
 apps/bos/4.asm | 139 +++++++++++++++++++++++++++------------------------------
 1 file changed, 65 insertions(+), 74 deletions(-)

diff --git a/apps/bos/4.asm b/apps/bos/4.asm
index 067ae95f..71c51d97 100644
--- a/apps/bos/4.asm
+++ b/apps/bos/4.asm
@@ -1,3 +1,5 @@
+; A boot sector that enters 32-bit protected mode.
+;
 ; To convert to a disk image:
 ;   cd apps/bos
 ;   nasm 4.asm -f bin -o boot.bin
@@ -6,32 +8,32 @@
 ; Or:
 ;   bochs  # bochsrc loads boot.bin
 
-; A boot sector that enters 32-bit protected mode.
 [org 0x7c00]
 
-  mov bp, 0x9000          ; Set the stack.
+  mov bp, 0x9000
   mov sp, bp
 
   mov bx, MSG_REAL_MODE
   call print_string
 
-  call switch_to_pm      ; Note that we never return from here.
+  call switch_to_pm
+  ; never gets here
 
   jmp $
 
-print_string:
-    pusha
-    mov ah, 0x0e
-loop:
-    mov al, [bx]
-    int 0x10
-    add bx, 1
-    cmp al, 0
-    jne loop
-    popa
-    ret
-
-; GDT
+switch_to_pm:
+  cli
+  lgdt [gdt_descriptor]
+  ; set LSB of CR0
+  mov eax, cr0
+  or eax, 0x1
+  mov cr0, eax
+  jmp CODE_SEG:init_pm  ; Far jump to a new segment containing 32-bit code.
+                        ; This also forces the CPU to flush its cache of
+                        ; prefetched and real-mode decoded instructions.
+  ; never gets here
+
+;; === GDT stuff
 gdt_start:
 
 gdt_null:  ; the mandatory null descriptor
@@ -50,7 +52,7 @@ gdt_code: ; the code segment descriptor
   db 11001111b  ; 2nd flags, Limit (bits 16-19)
   db 0x0        ; Base (bits 24 -31)
 
-gdt_data: ;the  data segment descriptor
+gdt_data: ; the  data segment descriptor
   ; Same as code segment except for the type flags:
   ; type flags: (code)0 (expand down)0 (writable)1 (accessed)0 -> 0010b
   dw 0xffff    ; Limit (bits 0-15)
@@ -60,28 +62,60 @@ gdt_data: ;the  data segment descriptor
   db 11001111b ; 2nd flags, Limit (bits 16-19)
   db 0x0       ; Base (bits 24 -31)
 
-gdt_end:       ; The reason for putting a label at the end of the
-               ; GDT is so we can have the assembler calculate
-               ; the size of the GDT for the GDT decriptor (below)
+gdt_end:
 
-; GDT descriptor
 gdt_descriptor:
-  dw gdt_end - gdt_start - 1  ; Size of our GDT, always less one
-                              ; of the true size
-  dd gdt_start                ; Start address of our GDT
-
-; Define some handy constants for the GDT segment descriptor offsets, which
-; are what segment registers must contain when in protected mode. For example,
-; when we set DS = 0x10 in PM , the CPU knows that we mean it to use the
-; segment described at offset 0x10 (i.e. 16 bytes) in our GDT, which in our
-; case is the DATA segment (0x0 -> NULL; 0x08 -> CODE; 0x10 -> DATA)
+  dw gdt_end - gdt_start - 1  ; last valid byte
+  dd gdt_start
+
+; some handy constants
 CODE_SEG equ gdt_code - gdt_start
 DATA_SEG equ gdt_data - gdt_start
+;; === end GDT stuff
 
+;; some 16-bit helpers
+
+; print string whose address is in bx
+print_string:
+  pusha
+  mov ah, 0x0e
+loop:
+  mov al, [bx]
+  int 0x10
+  add bx, 1
+  cmp al, 0
+  jne loop
+  popa
+  ret
+
+;; 32-bit code starts here
 [bits  32]
+
+; Initialise registers and the stack once in PM.
+init_pm:
+
+  mov ax, DATA_SEG        ; Now in PM, our old segments are meaningless,
+  mov ds, ax              ; so we point our segment registers to the
+  mov ss, ax              ; data selector we defined in our GDT
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+
+  mov ebp, 0x90000        ; Update our stack position so it is right
+  mov esp, ebp            ; at the top of the free space.
+
+  ;; Protected Mode is now initialized
+
+  mov ebx, MSG_PROT_MODE
+  call print_string_pm
+
+  ; all done; hang
+  jmp $
+
 ; Define  some  constants
 VIDEO_MEMORY  equ 0xb8000
 WHITE_ON_BLACK  equ 0x0f
+
 ; prints a null -terminated  string  pointed  to by EDX
 print_string_pm:
   pusha
@@ -100,53 +134,10 @@ print_string_pm_done:
   popa
   ret               ; Return  from  the  function
 
-[bits 16]
-; Switch to protected mode
-switch_to_pm:
-  cli                     ; We must switch of interrupts until we have
-                          ; set up the protected mode interrupt vector
-                          ; otherwise interrupts will run riot.
-
-  lgdt [gdt_descriptor]   ; Load our global descriptor table, which defines
-                          ; the protected mode segments (e.g. for code and data)
-
-  mov eax, cr0            ; To make the switch to protected mode, we set
-  or eax, 0x1             ; the first bit of CR0, a control register
-  mov cr0, eax
-
-  jmp CODE_SEG:init_pm    ; Make a far jump (i.e. to a new segment) to our 32-bit
-                          ; code. This also forces the CPU to flush its cache of
-                          ; prefetched and real-mode decoded instructions, which can
-                          ; cause problems.
-
-[bits 32]
-; Initialise registers and the stack once in PM.
-init_pm:
-
-  mov ax, DATA_SEG        ; Now in PM, our old segments are meaningless,
-  mov ds, ax              ; so we point our segment registers to the
-  mov ss, ax              ; data selector we defined in our GDT
-  mov es, ax
-  mov fs, ax
-  mov gs, ax
-
-  mov ebp, 0x90000        ; Update our stack position so it is right
-  mov esp, ebp            ; at the top of the free space.
-
-  call BEGIN_PM           ; Finally, call some well-known label
-
-[bits 32]
-; This is where we arrive after switching to and initialising protected mode.
-BEGIN_PM:
-
-  mov ebx, MSG_PROT_MODE
-  call print_string_pm    ; Use our 32-bit print routine.
-
-  jmp $                      ; Hang.
-
 ; Global variables
 MSG_REAL_MODE   db "Started in 16-bit Real Mode", 0
 MSG_PROT_MODE   db "Successfully landed in 32-bit Protected Mode", 0
+
 ; Bootsector padding
 times 510-($-$$) db 0
 dw 0xaa55
-- 
cgit 1.4.1-2-gfad0