From 4c9086e365e47eb5eee2d9c5f102dc71f8b504ab Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Sat, 14 Sep 2019 13:17:44 +0000
Subject: [PATCH] Rework bootloader to have adjustable memory layout

---
 boot/floppy.asm | 180 +++++++++++++++++++++++++-----------------------
 1 file changed, 94 insertions(+), 86 deletions(-)

diff --git a/boot/floppy.asm b/boot/floppy.asm
index 5ea7a72..1453f4e 100644
--- a/boot/floppy.asm
+++ b/boot/floppy.asm
@@ -1,18 +1,17 @@
-org 0x7600
-
 ; Memory layout:
-; 060:0000 PSP for loaded program
-; 060:0100 Loaded program
-; 060:7800 Our code (= 0:7C00)
-; 060:8000 FAT table
-; 060:E000 Root directory
-; 060:FF00 Stack area
+%define mbr     0x00600 ; 1 sector
+%define self	0x00800 ; 1 sector
+%define fattab  0x01000 ; variable size
+%define rootdir 0x06000 ; variable size
+%define psp     0x07B00 ; 0x100 bytes
+%define prog    0x07C00 ; 64k - psp
 
-init:
-  jmp 0x060:main
+org self
 
-times (0x0B - ($-$$)) db 0
 fdc:
+.jmp:
+  jmp short main
+  times (0x0B - ($-$$)) db 0
 .ss:
   dw 0x200	; sector size
 .sc:
@@ -50,6 +49,87 @@ fdc:
   db "FAT12"
   times (62 - ($-$$)) db " "
 
+main:
+  cli
+
+  ; Stack grows down from PSP + 64k
+  mov ax, (psp >> 4)
+  mov ss, ax
+  xor sp, sp
+
+  ; Relocate from [prog] to [self]
+  xor ax, ax
+  mov ds, ax
+  mov es, ax
+  mov si, prog
+  mov di, self
+  mov cx, 0x100
+  rep movsw
+
+  ; Jump into relocated code
+  jmp 0:.relocated
+.relocated:
+
+  mov [fdc.drv], dl ; save drive number in fd
+  sti
+
+  ; load fat table into memory
+  mov ax, [fdc.rsc]
+  mov cx, [fdc.sf]
+  xor dx, dx
+  mov bx, fattab
+  call load_sectors
+
+  ; calculate length of rootdir
+  mov ax, [fdc.rde]
+  mov cl, 4
+  shr ax, cl ; 32 bytes per entry
+  mov cx, ax
+
+  ; load root dir
+  xor dx, dx
+  mov ax, [fdc.sf]
+  mul byte [fdc.fn]
+  add ax, [fdc.rsc]
+  mov bx, rootdir
+  call load_sectors
+
+  ; remember where we left off
+  ; clusters start after rootdir
+  mov [cluster_offset], ax
+
+  call load_file
+  mov bp, 0x0032
+  jc error
+
+  ; get length of arguments
+  mov cx, 0x007F
+  mov di, arguments
+  xor ax, ax
+  repne scasb
+  dec di
+  sub di, arguments
+  mov cx, di
+
+  ; setup arguments field
+  mov [psp+0x080], cl
+  mov di, 0x081
+  mov si, arguments
+  rep movsb
+  mov BYTE [di], 0x0D
+
+  ; setup int 19h call at 0000, and push its address to stack
+  mov WORD [psp], 0x19CD
+  push sp
+
+  ; clear out registers and jump into target
+  xor ax, ax
+  xor cx, cx
+  xor dx, dx
+  xor bx, bx
+  mov dl, [fdc.drv]
+  jmp 0x07B0:0x0100
+
 ; Read sectors from disk
 ; Does not return on error
 ; ax and bx will be incremented, cx decremented
@@ -99,7 +179,7 @@ load_sectors:
 ; Load the file in [filename]
 ; or exit with carry set
 load_file:
-  mov si, 0xE000
+  mov si, rootdir
   mov cx, [fdc.rde]
 .loop:
   call file_match
@@ -127,7 +207,7 @@ file_match:
 read_clusters:
   add si, 0x1A
   lodsw
-  mov bx, 0x0100
+  mov bx, prog
 .loop:
   ; read cluster into area for target file
   push ax
@@ -144,7 +224,7 @@ read_clusters:
   mov si, ax
   shr si, 1
   add si, ax
-  add si, 0x8000
+  add si, fattab
 
   ; load entry from FAT, truncate to 12 bit
   mov dx, [si]
@@ -161,76 +241,6 @@ read_clusters:
 
   ret
 
-main:
-  cli
-  ; Everything in one segment, stack starts on top
-  mov ax, cs
-  mov ds, ax
-  mov es, ax
-  mov ss, ax
-  xor sp, sp
-  mov [fdc.drv], dl ; save drive number in fdc
-  sti
-
-  ; load fat table into memory
-  mov ax, [fdc.rsc]
-  mov cx, [fdc.sf]
-  xor dx, dx
-  mov bx, 0x8000
-  call load_sectors
-
-  ; calculate length of rootdir
-  mov ax, [fdc.rde]
-  dec ax
-  mov cl, 4
-  shr ax, cl ; 32 bytes per entry
-  inc ax
-  mov cx, ax
-
-  ; load root dir
-  xor dx, dx
-  mov ax, [fdc.sf]
-  mul byte [fdc.fn]
-  add ax, [fdc.rsc]
-  mov bx, 0xE000
-  call load_sectors
-
-  ; remember where we left off
-  ; clusters start after rootdir
-  mov [cluster_offset], ax
-
-  call load_file
-  mov bp, 0x0032
-  jc error
-
-  ; get length of arguments
-  mov cx, 0x007F
-  mov di, arguments
-  xor ax, ax
-  repne scasb
-  dec di
-  sub di, arguments
-  mov cx, di
-
-  ; setup arguments field
-  mov [0x080], cl
-  mov di, 0x081
-  mov si, arguments
-  rep movsb
-  mov BYTE [di], 0x0D
-
-  ; setup int 19h call at 0000, and push its address to stack
-  mov WORD [0000], 0x19CD
-  push sp
-
-  ; clear out registers and jump into target
-  xor ax, ax
-  xor cx, cx
-  xor dx, dx
-  xor bx, bx
-  mov dl, [fdc.drv]
-  jmp 0x0100
-
 error:
   mov ax, bp
   mov ah, 0x0e
@@ -245,8 +255,6 @@ error:
 cluster_offset:
   dw 0
 
-times (0x174 - ($-$$)) db 0
-
 filename:
   db "KERNEL  COM"