From f1c414bb26f39bb802295f24dd4a6dae4fbb12df Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Mon, 20 Dec 2021 20:08:37 +0000
Subject: [PATCH] implement bpb parsing

---
 src/@rdos.asm | 238 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 152 insertions(+), 86 deletions(-)

diff --git a/src/@rdos.asm b/src/@rdos.asm
index ccd2e3c..4d3c5de 100644
--- a/src/@rdos.asm
+++ b/src/@rdos.asm
@@ -6,24 +6,11 @@ cseg	equ 0x0
 %include "farptr.inc"
 
 	; dflags bitfields
-DRVLOG	equ 0x01 ; bit 0 (1) - drive logged in (see drvnum)
-DRVRST	equ 0x02 ; bit 1 (2) - controller configured
-DRVEXT	equ 0x04 ; bit 2 (4) - EBIOS supported
-DIRTY	equ 0x08 ; bit 3 (8) - dskbuf dirty
-
-	; initial setup for disk i/o
-	; copy previously set DPT to our data area
-dinit	xor ax, ax
-	mov es, ax
-	les bx, [es:4*0x1E]
-	mov dx, dpt
-	mov cx, 11
-	call lodfar
-	; set interrupt vector
-	mov es, cx
-	mov word [es:4*0x1E], dpt
-	mov word [es:4*0x1E+2], ds
-	ret
+DRVLOG	equ 0x01 ; bit 0 - drive logged in (see drvnum)
+DRVCTL	equ 0x02 ; bit 1 - controller configured
+DRVEXT	equ 0x04 ; bit 2 - EBIOS supported
+DIRTY	equ 0x08 ; bit 3 - dskbuf dirty
+DRVCHS	equ 0x10 ; bit 4 - CHS geometry known
 
 	align 16
 	; IBM Interrupt Sharing Protocol structure
@@ -52,8 +39,11 @@ int13	pushf
 	; read BIOS int13h/AH=8 values
 	; DPT data gets copied over our dpt
 	; CHS data gets inserted into our bpb
-getprm	test byte [dskflag], 4
+getprm	test byte [dflags], DRVEXT
 	jnz .ret
+	; fallback non-zero values (5 1/4", 160kB)
+	mov word [drvspt], 8
+	mov word [drvspc], 8
 	; do the query
 	mov ah, 8
 	mov dl, [biosnum]
@@ -65,12 +55,16 @@ getprm	test byte [dskflag], 4
 	jz .nochs
 	; get and store sector number
 	and cx, 0x3F
-	mov word [bpb+BPBSPT], cx
-	; get and store number of heads
+	mov word [drvspt], cx
+	; get and store number heads
 	xchg dl, dh
 	and dx, 0xFF
 	inc dx
-	mov [bpb+BPBNOS], dx
+	; multiply with sectors per track
+	mov ax, dx
+	mul cx
+	; store sectors per cylinder
+	mov [drvspc], ax
 	; test if DPT ptr is non-zero
 .nochs	mov ax, es
 	or ax, di
@@ -87,98 +81,128 @@ getprm	test byte [dskflag], 4
 	; IN	dl	drive number
 	; dont do anything if drive already selected
 select	cmp dl, [drvnum]
-	je logfdd.ret
+	je getprm.ret
 	; clear out current contents
 	push dx
 	call flush
 	mov ax, 0xFFFF
-	mov [dskseek], ax
-	mov [dskseek+2], ax
-	mov byte [dskflag], 0
+	mov [drvseek], ax
+	mov [drvseek+2], ax
+	and word [dflags], ~(DRVLOG|DRVCTL|DRVEXT|DIRTY|DRVCHS)
 	pop dx
 	; set current drive number
-	mov [dsknum], dl
+	mov [drvnum], dl
 	; fork off if hdd
 	cmp dl, 2
 	jnc loghdd
 
 logfdd	mov [biosnum], dl
-	; assume some default values
-	xor ax, ax
-	mov word [bpb+BPBHS], ax
-	mov word [bpb+BPBHS+2], ax
-	mov word [bpb+BPBNOS], 2 ; 5 1/4" 360kB
-	mov word [bpb+BPBSPT], 9
-	; reset dpt to defaults
+	and dx, 0x0D
+	; query bios for geometry and DPT
 	call getprm
-	; load boot sector
+	; assume 512 bytes per sector
+	; TODO: read this from DPT
+	mov byte [drvss], 2
+	; fill partition offset with zeroes
 	xor ax, ax
-	xor dx, dx
-	call mapabs
-	; copy bios parameter block
-	; TODO: guess from first byte of FAT if BPB invalid
-	call rstseg
-	mov si, dskbuf+BPBOFF
-	mov di, bpb
-	mov cx, BPBSIZ
-	rep movsb
-	; copy SPT to DPT
-	mov al, [bpb+BPBSPT]
-	mov [dpt+4], al
-.ret	ret
+	mov di, drvoff
+	stosw
+	stosw
+	; fill partition end with 1's
+	not ax
+	stosw
+	stosw
+	; shared code with loghdd
+	jmp rdbpb
 
-loghdd	sub dl, 2
-	cmp dl, 4
-	jnc logerr
-	push dx
-	mov dl, 0x80
+	; Log in a drive on a hard disk
+	; IN	dl	bits 0-1: partition number
+	;		bits 2-7: drive number
+	; Partition offset is read from MBR
+	; split drive & partition number
+loghdd	mov dh, dl
+	and ah, 0x3
+	shr dl, 1
+	shl dl, 1
+	add dl, 0x80
+	; partition number in DH
 	mov byte [biosnum], dl
-	; test for ebios extensions
+	push dx
+	; test for EBIOS extensions
 	mov ah, 0x41
 	mov bx, 0x55AA
 	call int13
 	sbb bx, 0xAA55
-	jnz .chs
-	; enable ebios and assume controller is configured
-	or byte [dflags], DRVCTL | DRVEXT
-	; get chs data (needed or we cant load vbr)
-.chs	call getprm
-	; read mbr
+	jnz .noeb
+	; enable EBIOS and assume controller is configured
+	or word [dflags], DRVCTL | DRVEXT
+	; hard disks are always assumed to be 512 bytes / sector
+.noeb	mov byte [drvss], 2
+	; query BIOS for geometry
+	call getprm
+	or word [dflags], DRVCHS
+	; read MBR
 	xor ax, ax
 	xor dx, dx
-	call mapabs
+	call map
 	; get partition number
 	pop bx
 	xor bh, bh
 	mov cl, 4
 	sal bx, cl
-	; bail out if no partition
+	; bail out if partition undef
 	cmp byte [dskbuf+0x1be+bx+4], 0
-	je logerr
+	je .ret
 	; load partition offset
 	mov ax, [dskbuf+0x1be+bx+8]
 	mov dx, [dskbuf+0x1be+bx+8+2]
-	; save to to stack
-	push dx
-	push ax
-	; load vbr
-	call mapabs
-	; copy bpb
-	mov si, dskbuf+BPBOFF
-	mov di, bpb
-	mov cx, BPBSIZ
-	rep movsb
-	; fix CHS data
-	call getprm
-	; fix partition offset
-	pop word [bpb+BPBHS]
-	pop word [bpb+BPBHS+2]
-	;pop [bpb+BPBNOS]
-	;pop [bpb+BPBSPT]
-	ret
+	; store partition offset
+	mov [drvoff], ax
+	mov [drvoff+2], dx
+	; add partition length
+	add ax, [dskbuf+0x1be+bx+12]
+	adc dx, [dskbuf+0x1be+bx+12+2]
+	; partitions after 4 TB mark not allowed
+	jc .ret
+	; store partition end
+	mov [drvend], ax
+	mov [drvend+2], dx
+.ret	ret
 
-logerr	stc
-	ret
+	; parse bios parameter block from drvoff
+	; drvoff and biosnum need to be set up
+rdbpb	mov ax, [drvoff]
+	mov dx, [drvoff+2]
+	call map
+	; read partition start
+	mov ax, [drvoff]
+	mov dx, [drvoff+2]
+	; add reserved sectors
+	add ax, [dskbuf+BPBOFF+BPBRSC]
+	adc dx, 0
+	jc .ret
+	; save offset of fat table
+	mov [drvfat], ax
+	mov [drvfat+2], dx
+	; calculate length of all fat tables
+	mov ah, 0
+	mov al, [dskbuf+BPBOFF+BPBFN]
+	mov dx, [dskbuf+BPBOFF+BPBFS]
+	mov [drvfn], dx
+	mul dx
+	add ax, [drvfat]
+	adc dx, [drvfat+2]
+	; save offset of root directory
+	mov [drvrd], ax
+	mov [drvrd+2], dx
+	; calculate length of root directory
+	mov ax, [dskbuf+BPBOFF+BPBRDE]
+	mov cl, [drvss]
+	add cl, 2
+	shr ax, cl
+	; read offset to cluster area
+	; read cluster size
+.ret	ret
 
 	; map sector into dskbuf
 	; skip doing a read if sector number matches
@@ -227,13 +251,13 @@ diskio	mov cl, 1 ; read len
 	mov si, dap
 	jmp .do
 	; check if we can skip controller reset
-.noext	test word [dflags], DRVRST
+.noext	test word [dflags], DRVCTL
 	jnz .norst
 	; do controller reset
 	mov dl, [biosnum]
 	mov ah, 0
 	call int13
-	or word [dflags], DRVRST
+	or word [dflags], DRVCTL
 	; put linear sector num into dx:ax
 .norst	mov ax, [drvseek]
 	mov dx, [drvseek+2]
@@ -272,16 +296,58 @@ diskio	mov cl, 1 ; read len
 	clc
 	ret
 	; assume controller is misconfigured
-.err	and word [dflags], ~DRVRST
+.err	and word [dflags], ~DRVCTL
 	; exit with carry flag set
 	stc
 	ret
 
+	; Get interrupt vector
+	; IN	al	interrupt number
+	; OUT	es:bx	vector
+getint	mov bl, al
+	mov bh, 0
+	; multiply with 4
+	add bx, bx
+	add bx, bx
+	les bx, [cs:bx]
+	ret
+
+	; Set interrupt vector
+	; IN	al	interrupt number
+	;	ds:dx	vector
+setint	mov bl, al
+	mov bh, 0
+	; multiply with 4
+	add bx, bx
+	add bx, bx
+	mov [cs:bx], dx
+	mov [cs:bx+2], ds
+	ret
+
 init	mov ax, cs
 	; boot failure if loaded wrong
 	cmp ax, cseg
 	je $+4
 	int 0x18
+	call rstseg
+	; fetch current DPT
+	les bx, [4*0x1E]
+	mov dx, dpt
+	mov cx, 11
+	call lodfar
+	; set interrupt vector
+	mov word [4*0x1E], dpt
+	mov word [4*0x1E+2], ds
+	; save int 13 handler
+	les bx, [4*0x13]
+	mov [i13isp+2], bx
+	mov [i13isp+4], es
+	; install ours
+	mov word [4*0x13], int13i
+	mov [4*0x13+2], cs
+
+	mov ax, 0x0e37
+	int 0x10
 halt	hlt
 	jmp halt