ORG 7C00H
	USE16

BPBBEG:
	JMP  SHORT BPBPEND
	NOP

		DB "PEDOS1.0"
BYTSPERSEC:	DW 512		; total bytes per sector on this volume
SECPERCLUS:	DB 1		; total sectors per cluster on this volume
RSVDSECCNT:	DW 1		; unused
NUMFATS:	DB 2		; total FATs on this volume
ROOTENTCNT:	DW 224		; total entries in the Root Directory
		DW 80 * 36
		DB 240
FATSZ16:	DW 9		; total sectors per FAT on this volume
SECPERTRK:	DW 18		; total sectors per track on this idks
NUMHEADS:	DW 2		; total heads per cyliner on this idks
		DD 0
		DD 0
		DB 0
		DB 0
		DB 41
		DD 0
		DB "           "
		DB "FAT12   "
;
; PEDOS MEMORY MAP
;
;   -  --  ROM / VIDEO  -- A000:0
;   -   -----------------
;   -  --  BIOS DATA    -- 8000:0
;   -   -----------------
;   -  --  FREE         -- ????:?
;   -   -----------------
;   -  --  BOOT / FREE  -- 0000:7C00
;   -   -----------------
;   -  --  FREE         -- ????:?
;   -   -----------------
;   -  --  DRIVERS      -- ????:?
;   -   -----------------
;   -  --  SYSTEM       -- 0050:0
;   -   -----------------
;   -  --  BIOS BATA    -- 0000:400
;   -   -----------------
;   -  --  IVT          -- 0000:0
;

;
; INITIALIZE SEGMENT REGISTERS
;
BPBPEND:
	XOR  AX, AX
	MOV  DS, AX
	CLI
	PUSH DS
	POP  SS
	MOV  SP, 7C00H
	PUSH DS
	PUSH CHKDSK
	STI
	RETF	; SET CS TO KNOWN VALUE
;
; LOAD ROOT DIRECTORY AND CHECK FOR
; SYSTEM FILE
;
CHKDSK:
	MOV  AX, WORD BUFFER
	SHR  AX, 4
	MOV  ES, AX	; ES = 07E0
	XOR  AX, AX
	MOV  AX, WORD [FATSZ16]
	MUL  WORD [NUMFATS]
	ADD  AL, BYTE [RSVDSECCNT]
	mov  bx, 0x7e00
	MOV  DI, 1
	CALL READSEC


;	mov  ah, 14
;	mov  al, '/'
;	int  16
;	jmp  $

	LEA  SI, [FILE]	; ADDRESS OF FILENAME
	MOV  DI, 0
	MOV  CX, 11		; 11 BYTES PER FAT FILENAME
	CLD
	REPZ CMPSB
	JZ   LOADFILE
	JMP  ERROR

; DOSBIOS.SYS CONFIRMED: LOAD THE
; FILE INTO MEMORY.

LOADFILE:
	mov  ah, 14
	mov  al, '.'
	int  16
	jmp  $

	MOV  AX, 32
	MOV  BX, WORD [ROOTENTCNT]	; TOTAL FATS ON DISK
	MUL  BX
	MOV  BX, WORD [BYTSPERSEC]	; FAT SIZE IN SECTORS
	DIV  BX
	;
	; AX = SIZE OF ROOT DIRECTORY
	;   IN SECTORS
	;
	ADD  AX, BP	; SIZE + LOCATION = DATA REGION
	POP  BX
	PUSH BX
	MOV  DI, 1	; DOS SIZE IS HARD CODED FOR NOW - MAY
	CALL READSEC	; CHANGE IN THE FUTURE
RUNDOS:
	POP  BX
	PUSH 0
	PUSH BX
	RETF
;
; READ THE SPECIFIED SECTORS INTO MEMORY
; AT LOGICAL ES:BX
;
; IN:	DX:AX	= HEAD, TRACK, SECTOR NUMBER
;	DI	= SECTOR COUNT
;
READSEC:
	PUSHA
	DIV  WORD [SECPERTRK]
	;
	; AX = LBA / SECPERTRACK
	; DX = LBA % SECPERTRACK
	;
	MOV  CX, DX
	INC  CX		; CX = LBA 1
	XOR  DX, DX
	DIV  WORD [NUMHEADS]
	;
	; AX = (LBA / SECPERTRACK) / CYLHEADCNT = CYLINDER
	; DX = (LBA / SECPERTRACK) % CYLHEADCNT = HEAD
	;
	MOV  CH, AL
	SHL  AH, 6
	OR   CL, AH
	MOV  DH, DL
	MOV  DL, 0
	;
	; DH = HEAD HUMBER
	; DL = DRIVE
	;
	MOV  AH, 2
	MOV  AL, 1
	INT  19
	JNC  NEXTSEC	; IN CASE OF ERRORS, FOLLOW
ERROR:
	LEA  SI, [ERRMSG]	; SOMETHING WENT WRONG, SO THROW AN ERROR
;
; WRITE SPECIFIED STRING TO VIDEO
; MEMORY
;
; DS:SI = ADDRESS OF STRING
;
PRINT:
	LODSB
	OR   AL, 0
	JZ   HALT
	MOV  AH, 14
	MOV  BX, 7
	INT  16
	JMP  PRINT
HALT:
	MOV AX, 0
	INT 22
	INT 25
;
; CONTINUE 'READSEC'
;
NEXTSEC:
	POPA
	DEC  DI
	JZ   RETURN	; SKIP PRINT SUBROUTINE
	ADD  BX, WORD [BYTSPERSEC]
	ADD  AX, 1
	ADC  DX, 0
	JMP  READSEC
RETURN:
	RET
;
; KEEP DATA BELOW CODE, UNTIL SECTOR
; MARKER
;
ERRMSG:	DB 10, "Disk Error or DOSBIOS.SYS is missing.", 13
	DB 10, "Press any key to restart.", 13, 10, 10, 0
FILE:	DB "DOSBIOS SYS"

	TIMES (512 - 2) - ($ - $$) DB 0

MARKER:	DW 0AA55H
BUFFER:
;
; THE SYSTEM IS LOADED DIRECTLY AFTER
; BOOTSTRAP
;