From 47630b2c0338570ef28248374dbfee32bdb58ab3 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Mon, 28 Oct 2019 10:38:18 +0000
Subject: [PATCH] Implement boot based on reserved sectors

---
 Makefile      |  11 ++--
 bios/8086.asm |   7 ++-
 boot/fat.asm  | 140 +++++---------------------------------------------
 3 files changed, 23 insertions(+), 135 deletions(-)

diff --git a/Makefile b/Makefile
index 47bf6d4..9b53c51 100644
--- a/Makefile
+++ b/Makefile
@@ -2,8 +2,11 @@ FLOPPY = 360
 
 RAM = 0x10000
 
+BDOSIMG = bdos.bin
+BIOSIMG = bios8086.bin
+
 PROGRAMS = $(patsubst %.asm,%.com,$(wildcard com/*.asm))
-DISTFILES = bdos.bin $(PROGRAMS)
+DISTFILES = $(PROGRAMS)
 ROMS =
 
 QEMU_ARCH = $(shell uname -m)
@@ -52,8 +55,10 @@ com/%.com: com/%.asm
 bios%.bin: bios/%.asm
 	$(NASM) $(NASM_ARGS) -o $@ $<
 
-fdimage.img: boot/fat.bs $(DISTFILES)
-	mformat -C -i $@ -f $(FLOPPY) -B boot/fat.bs ::
+fdimage.img: boot/fat.bs $(BDOSIMG) $(BIOSIMG) $(DISTFILES)
+	mformat -R 9 -C -i $@ -f $(FLOPPY) -B boot/fat.bs ::
+	dd if=$(BDOSIMG) bs=512 seek=1 count=4 conv=notrunc of=$@
+	dd if=$(BIOSIMG) bs=512 seek=5 count=4 conv=notrunc of=$@
 	mcopy -i $@ $(DISTFILES) ::
 
 hdimage.img: boot/mbr.bs fdimage.img
diff --git a/bios/8086.asm b/bios/8086.asm
index fbd41b6..1c53f79 100644
--- a/bios/8086.asm
+++ b/bios/8086.asm
@@ -23,13 +23,12 @@ JMP NEAR WRITE
 JMP NEAR LISTST
 
 BOOT:
-  MOV AL, 0x19
+  MOV AL, 0x86
   CALL MSG
-  DB "BIOS 8086", 0
-  RET
+  DB "BIOS", 0
 
 WBOOT:
-  STC
+  JMP (RAM - 0x1000)
   RET
 
 ; CHAR I/O
diff --git a/boot/fat.asm b/boot/fat.asm
index 71a2ab4..ccb2cbf 100644
--- a/boot/fat.asm
+++ b/boot/fat.asm
@@ -1,18 +1,11 @@
-; Memory layout:
 %define segment   0x00100
-
 %define self      (0x7C00-(segment<<4)) ; 1 sector
-%define fattab    (self+0x200) ; variable size
-%define rootdir   0x00100 ; variable size
-%define prog      0x0F000 ; 4K at the end for OS
+%define prog      (RAM-0x1000) ; 4K at the end for OS
 
+cpu 8086
 org self
-
 jmp short init
 
-cluster_offset:
-  dw 0
-
 times (0x0B - ($-$$)) db 0
 
 fdc:
@@ -70,122 +63,11 @@ main:
   mov es, ax
 
   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
-
-  ; Load kernel
+  mov ax, 1
+  mov cx, 8
+  mov dx, 0
   mov bx, prog
-  mov ax, kernel_name
-  call load_file
-  mov bp, 0x3332
-  jc error
-
-  mov dl, [fdc.drv]
-
-  ; jump into kernel
-  jmp segment:prog
-
-; Load a file into memory
-; IN	AX	pointer to 8.3 filename
-;	ES:BX	pointer to target area
-; OUT	CF	flag set if error
-;	DI	file size in bytes (<64K)
-load_file:
-  mov si, rootdir
-  mov cx, [fdc.rde]
-.search:
-  push cx
-  push si
-  mov di, ax
-  mov cx, 11
-  repe cmpsb
-  pop si
-  pop cx
-  je .read
-  add si, 0x20
-  loop .search
-  xor di, di
-  stc
-  ret
-.read:
-  mov ax, [si+0x1A]
-  mov di, [si+0x1C]
-  jmp read_clusters
-
-; Read the file given by cluster number
-; into the target program area
-; in	ax	cluster number
-read_clusters:
-  ; read cluster into area for target file
-  push ax
-  sub ax, 2
-  mul BYTE [fdc.sc]
-  add ax, [cluster_offset]
-  xor dx, dx
-  mov cx, [fdc.sc]
-  xor ch, ch
-  call load_sectors
-  pop ax
-
-  ; calculate index in FAT
-  mov si, ax
-  shr si, 1
-  add si, ax
-  add si, fattab
-
-  ; load entry from FAT, truncate to 12 bit
-  mov dx, [si]
-  test ax, 1
-  jz .noshift
-  mov cl, 4
-  shr dx, 4
-.noshift:
-  mov ax, dx
-
-  and ax, 0x0FFF
-  cmp ax, 0x0FF8
-  jc read_clusters
-
-  ret
-
-; Read sectors from disk
-; Does not return on error
-; ax and bx will be incremented, cx decremented
-; in    dx:ax   sector number
-;       es:bx   buffer
-;	cx	number of sectors to read
-; out	dx:ax	next sector to read
-;	es:bx	next free buffer
-;	cx	zero
-load_sectors:
-  ; fail instantly if reading sectors > 16 bit
-  test dx, dx
-  mov bp, 0x3330
-  jnz error
 
 .loop:
   push ax
@@ -217,10 +99,15 @@ load_sectors:
 
   ; count up for next sector
   add bx, 0x0200
-  inc ax
+  add ax, 1
+  adc dx, 0
 
   loop .loop
-  ret
+
+  mov dl, [fdc.drv]
+
+  ; jump into kernel
+  jmp segment:(prog+0x0800)
 
 error:
   mov ax, bp
@@ -233,9 +120,6 @@ error:
   int 0x16
   int 0x19
 
-kernel_name:
-  db "BDOS    BIN"
-
 times (0x1FE - ($-$$)) db 0
 
 dw 0xAA55