; $Id: criter.asm 1574 2011-04-22 16:48:58Z bartoldeman $
;
;		Critical Error handler -- module
;
; Three macros to customize the assembler process:
;		COMPILE_COM in order to generate a .COM file (Debugging ONLY)
;			automatically defines COMPILE_STRINGS
;			includes this:	write(open("A:\AUX", O_RDWR), 0, 65535)
;			it is assumed that A: is empty first, then a floppy is
;			inserted and no AUX: device is present
;			--> produces 1st a block device Criter, then a char device
;				Criter
;		COMPILE_STRINGS to include English response strings right into
;			this image
;		NO_RESOURCE_BLOCK omit resource block at the end of the file
;		XMS_SWAP_CRITER disables interfering stuff when assembling for
;			XMS Swap variant
;
;	Doing a ECHO >A:\FILE results in:
;		WinNT 4 DOSbox:
;			no floppy in drive --> AX := 0x1800 ; [BP:SI+4] == 0x0000
;			write protected floppy & file already exists
;				--> AX := 0x98FF; [attr] == 0x8000
;			write protected floppy, but files does NOT exist
;				--> AX := 0x1800; [attr] == 0x0000
;		Win98 plain:
;			no floppy in drive --> AX := 0x1A00; [attr] == 0x08C2
;			write protected floppy -> AX := 0x1B00; [attr] == 0x08C2
;		FreeDOS build 2021:
;			no floppy in drive --> AX := 0x3800; [attr] == 0x08C2
;			write protected floppy -> same as above
;		FreeDOS build 2022:
;			no floppy in drive --> AX := 0x3800; [attr] == 0x08C2
;			write protected floppy -> stalled quite some time, but NO CRITER!
;		FreeDOS build 2023b [Mar 27 2001 21:51:20]:
;			no floppy in drive --> AX := 0x3800; [attr] == 0x08C2
;			write protected floppy -> stalled quite some time, but NO CRITER!
;
;	This handler does:
;	0) The current context is passed into this function within ES:BX
;		The original ES & BX are restored by CRITER.
;	1) Probe for Autofail <-> return FAIL for all criterrs
;	2) Display error message:
;	block devices:
;		Error "reading from"|"writing to" drive ?: XXX area: error
;	character devices:
;		Error "reading from"|"writing to" device XXXXXXX: error
;	3) Display user action possebilities:
;		(A)bort (I)gnore (R)etry (F)ail? _
;	4) Get user input & return to DOS (via IRET)
;
;	Fields of context used:
;		offset 0, byte (boolean), autofail
;
;	== What to do if neither I,R,nor F are allowed??
;	== Simply return??
;
;	Organization of the strings:
;	+ There may be upto 256 strings, numbered from 0 through N
;	+ The actual "error strings" must be the last ones and their
;	  order must be the same as the code in DI
;	+ All strings are packed together as follows:
;	  (N+1) words	near pointer to character sequence of string #X
;	  character sequences of strings, no obvious order among them required
;	  This block is created by an external tool, like the messages for
;	  FreeCom itself and simply attached to the criterr handler code.
;	+ Within the code procedure ?oString maps BL -> near pointer (BX)
;	  and displays the associated character sequence with ?oBuffer
;	+ Special sequences:
;		%%	--> a single percent sign
;		%#	--> where '#' is a decimal digit --> insert string
;				with number BYTE [?strarg#]
;		%A	--> where 'A' is a capital Latin letter --> insert
;				the buffer at ?strargA
;
;	Possible NLS hazards:
;	+ drive letter is created via "+ 'A'"
;	+ for robustness when displaying the driver's name, characters less
;	  than ' ' are ignored (usually control characters)
;
; $Log$
; Revision 1.5  2004/09/13 18:59:39  skaus
; add: CRITER: Repeat check autofail magic {Tom Ehlert/Eric Auer}
;
; Revision 1.4  2003/10/18 10:55:24  skaus
; bugfix: CRITERR: to use DOS API {Tom Ehlert/Bart Oldeman}
;
; Revision 1.3  2003/08/03 16:00:57  skaus
; bugfix: /F (AutoFail) for the XMS_Swap variant
;
; Revision 1.2  2002/04/02 18:09:31  skaus
; add: XMS-Only Swap feature (FEATURE_XMS_SWAP) (Tom Ehlert)
;
; Revision 1.1  2001/04/23 21:35:41  skaus
; Beta 7 changes (code split)
;
; Revision 1.1.2.3  2001/02/27 18:27:06  skaus
; fix: criter.asm: DEBUG does no longer compile to .COM (for consistency)
; chg: removed all NULLs to prevent NULL problematic, functions must be
; 	used fully prototyped now
;
; Revision 1.1.2.2  2001/02/18 17:59:36  skaus
; bugfix: KSSF: restore parentPSP on exit
; chg: Using STRINGS resource for all non-interactive messages
; chg: moving all assembly files to NASM
;
; Revision 1.1.2.1  2001/01/28 21:23:55  skaus
; add: Kernel Swap Support
;
; Revision 1.1.2.1.2.1  2000/12/17 21:57:36  skaus
; intermediate update 1
;

;;; Settings for stand-alone variant
%define COMPILE_STRINGS		;; always keep this enabled in this release!!
%define INCLUDE_STRINGS		;; use STRINGS.INC instead of hard-coded strings
;; %define AUTO_FAIL		;; make the autofail variant of Criter
%define HIDE_CRITER_DRIVES 26	;; For how many drives hide-multiple is active

;; Version of this module
MODULE_VERSION EQU 2

%include "../include/stuff.inc"
%ifndef XMS_SWAP_CRITER
%include "resource.inc"
%endif

???start:

%ifdef COMPILE_COM
ORG 100h

mov ax, cs
mov ds, ax
mov ax, 2524h
mov dx, _lowlevel_err_handler
int 21h

;; Trying to open a file on drive A:
mov ax, 3d02h
mov dx, dummy_file
int 21h

;; Trying to copy stuff on PRN:
mov bx, ax
mov ah, 40h
mov cx, 0ffffh
mov dx, 0
int 21h

mov ax, 4c00h
int 21h
int 20h

dummy_file DB "a:\ux", 0

%ifndef COMPILE_STRINGS
%define COMPILE_STRINGS
%endif
%endif

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; Real start
;; Join both modules into the same memory image in order to handle
;; them easier

%ifndef XMS_SWAP_CRITER
	;; Static context of KSwap support
%include "context.def" 

%include "dmy_cbrk.asm"
%endif

;; Low level Critical Error handler
;; Note: Both I/O functions should use the channel to read/write
;;	the characters. In this case: the BIOS.
;; Sidenote: RBIL states that some BIOSes destroy the BP register.
%macro printALtoConsole 0
	call ?oChar
%endmacro
%macro readALfromConsole 0
	mov ax, 0c07h	; clear buffer, read from STDIN one key without echo
	int 21h
%endmacro


LOCAL_BELL	EQU 7

;; Return values
IGNORE		EQU 0
RETRY		EQU 1
ABORT		EQU 2
FAIL		EQU 3

; bitmask of the "allowed action" flags
CodeIgnore	EQU 32
CodeRetry	EQU 16
CodeFail	EQU 8

;; String numbers -- fixed order section
; 0 -> %1: write to
; 1 -> %1: read from 

StrBlockDevice	EQU 2
StrCharDevice	EQU 3

StrArea EQU 4
; StrArea (area of failure)
; + 0 -> DOS
; + 1 -> FAT
; + 2 -> root
; + 3 -> data

; StrActionStrings 8
; + 0 -> Ignore
; + 1 -> Retry
; + 2 -> Abort
; + 3 -> Fail
StrIgnore	EQU 8
StrRetry	EQU 9
StrAbort	EQU 10
StrFail		EQU 11

StrQuestionMark	EQU 12
StrDelimiter	EQU 13
StrKeys			EQU 14		; enumerated valid input keys
		; format:	LK..KA..A
		;	L: BYTE number of K's (== number of A's)
		;	K: BYTE one keystroke
		;	A: BYTE one action code 0..3 (IRAF)
		;	a K's action code is located at the byte pointed to
		;	by the address of K plus L

StrErrCodes		EQU 15

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;; Entry point

; arguments:
;		ES:BX == pointer to context, package int24
;		else: as normal INT-24 handler

%ifdef XMS_SWAP_CRITER
	global _autofail_err_handler
_autofail_err_handler:
	mov al, FAIL
	iret
%endif

	global _lowlevel_err_handler
_lowlevel_err_handler:
%ifdef AUTO_FAIL
	;; most simple <-> return AL := 3
	mov al, FAIL
	iret
%else
	push dx
	pushm es, ds, bp, si, di, cx, bx, ax

	mov cx, cs
	mov ds, cx		; DS := local code/data segment

%ifdef DEBUG
	call ??dispAX
%endif

	push ax			; save AH bit 7  and AL for later

			;; free AL
	add al, 'A'		; AL may contain the drive number
	mov BYTE [??strargA], al	;; will be overwritten, if char device
	xor al, al
	mov BYTE [??strargA + 1], al	; end of string

	mov es, cx		; still shared local code/data segm

	shr ah, 1		; Carry := 0-> read; 1->write
	adc al, al		; AL := 0-> read; 1->write
	mov BYTE [??strarg1], al

	mov al, ah
	and al, 3		; AL := 0/1/2/3 -> DOS/FAT/root/data area
	add al, StrArea	;  make it a string#
	mov BYTE [??strarg2], al	;; will be ignored if char device

;;
;; and al, (CodeIgnore or CodeRetry or CodeFail) / 2
;; jz ONLY_ABORT_ALLOWED_AND_NOW??
;;
	mov al, ah
	and al, CodeFail / 2
	mov [??allowFail], al
	mov al, ah
	and al, CodeRetry / 2
	mov [??allowRetry], al
	and ah, CodeIgnore / 2
	mov [??allowIgnore], ah

;; AX is empty now
%ifdef DEBUG
	push ds
	mov ax, bp
	mov ds, bp
	push si
	push di
	push cx
	mov cx, 8
	mov di, ??_c
	add si, 10
??__e:
	lodsb
	cmp al, 32
	jnc ??__d
	mov al, 32
??__d:
	stosb
	loop ??__e
	pop cx
	pop di
	pop si
	mov ax, [si+4]
	pop ds
	call ??dispAX
	mov bx, ??_d
	call ?oBuffer
%endif

%if 0		;; No need when doing I/O through DOS
	mov ah, 0fh		; Get current video mode
	int 10h
	mov BYTE [??actPage], bh
	mov BYTE [??actColour], 255
%endif

	mov ax, di		; AL := lobyte(DI) -> error number
	add al, StrErrCodes
	mov BYTE [??strarg3], al

	mov bl, StrBlockDevice		; by default issue block device error
	pop ax						; AL := drive letter again
	shl ah, 1					; get bit 7 --> carry flag
	jnc ?noCharDevice

	mov ds, bp					; is still segment of device driver
	test BYTE [si+5], 128		; bit 7 == 1 if block device
	jz ?noCharDevice
			; fetch the name of the character device
		mov cx, 8		; max eight characters to display
		add si, 10		; located at offset 10
		mov di, ??strargA
		mov ah, ' '
		cld
?drvNameGetLoop:
		lodsb
		cmp al, ah
		jnc ?charOK
		mov al, ah
?charOK:
		stosb
		loop ?drvNameGetLoop

			;; Remove trailing whitespaces
		mov cx, 8		; max eight characters to display
		mov al, ah
		std
		dec di			; stosb leaves DI behind the last written byte
		repe scasb
		mov BYTE [ES:di+2], ch	; place termination character
		mov cx, cs
		mov bl, StrCharDevice		; issue device driver message
		mov al, 0ffh		; AL := invalidate

?noCharDevice:

	cld				; forward direction
	mov ds, cx		; CX is still or again == CS

	push bx
	;; Now many registers are available to use
	;; Perform the repeat check; AL == -1 (char device), 0..31 if block
	mov bx, ?repCheck
	mov di, dummyByte	; Tha byte at [DI} will be decremented eventually
	inc WORD [BX]	; check if enabled
	jz ?noRepCheck
	;; paranoid check to avoid memory overflow
	;; also skips if char device
	cmp al, HIDE_CRITER_DRIVES	; 0..26 == A..Z
	jnc ?noRepCheck
	cbw				; AH := 0
	add al, 2		; correction as BX is two bytes below
	add ax, bx		; repCheckByte area 
	mov di, ax
	inc BYTE [DI]	; will display the 1st and every 256th Criter
	jnz	?fail		; displayed already --> AutoFail && keep incremented [BX]
					; NOTE: There is no output generated til now!!
					;	The output channels have not been touched
?noRepCheck:
	dec WORD [BX]	; keep the word at value -1
	mov WORD [repCheckDecAddr], di

;; Try to find a suitable I/O channel
	mov ah, 62h		; Get PSP
	int 21h
	mov es, bx
;;	mov cx, [ES:32h]	; number of entries in JFT
	les bx, [ES:34h]	; Pointer to JFT
	mov al, [es:bx+2]	; stderr
	; patch STDIN & STDOUT to point to the found channel
	mov ah, al
	xchg WORD [ES:bx], ax
	mov WORD [?orgIOchannels], ax
	mov WORD [??repatchAddr], bx
	mov WORD [??repatchAddr+2], es
	pop bx
	mov es, cx			;; reset ES to the shared code/data segement


	call ?oString	; Display the Critical Error message string
	call ?newline
	mov bl, StrAbort
	call ?oString
	mov bl, 0		; try Ignore
	call ?oAction
	mov bl, 1		; try Retry
	call ?oAction
	mov bl, 3		; try Fail
	call ?oAction
	mov bl, StrQuestionMark
	call ?oString
	jmp short ?inputLoop

?fail:
	pop bx		; correct the stack from repeat check
	mov bl, FAIL
	jmp short ?iret	; actually return

?inputError:
	mov al, LOCAL_BELL
	printALtoConsole

?inputLoop:
	; Prepare to decode the character
	mov di, [StrKeys * 2 + 1 + ??strings]	; address of enumerated valid keys
	xor ax, ax
	mov al, [di]
	inc di
	mov cx, ax			; number of enumerated keys
	mov bx, ax

	readALfromConsole
	;; special keys (AL == 0) are ignored

;; Decode user input
	repne scasb
	jne ?inputError		; key not found

	mov bl, [bx+di-1]	; action code (DI is one byte too far by SCASB)
	mov al, [bx+??allow]
	or al, al
	jz ?inputError		; not allowed

?errRet:
	;; allowed action code in BL
	call ?newline

	;; Restore the JFT
	les di, [??repatchAddr]
	mov ax, 1234h
?orgIOchannels EQU $-2
	mov WORD [ES:di], ax

?iret:
	pop ax			; preserve AH
	mov al, bl		; action code
	cmp al, ABORT
	je ?iretNow
	cmp al, FAIL
	je ?iretNow
	;; AutoFail is activated for ABORT and FAIL only
	;; as user selected something else, reset the AutoFail state of the drive
	dec BYTE [1234h]
repCheckDecAddr EQU $-2
?iretNow:
	popm es, ds, bp, si, di, cx, bx
	pop dx
	iret

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;; procedures

;; dump allowed action #AL

?oAction:
	mov bh, 0
	or bh, [bx+??allow]
	jz ?oRet			; not allowed
	add bl, '5'			; local strarg code
	mov BYTE [??input+3], bl	; construct the temp output buffer
	mov bx, ??input
	jmp short ?oBuffer

;; dump string #BL onto screen
;; IN: BL = string number
;; OUT: BX destroyed

?oString:
	;; make sure BL is within limit
	mov al, BYTE [??strings]
	cmp bl, al
	jc ?oS_idOK
	mov bl, al
?oS_idOK:
	mov bh, 0
	add bx, bx
	mov bx, WORD [bx+??strings+1]
	jmp short ?oBuffer

;; dump character sequence pointed to by BX onto screen
;; IN: BX = address
;; OUT: BX destroyed

?oB_bufA:		;; Dump string argument A
	push bx
	mov bx, ??strargA
	call ?oBuffer
	jmp short ?oB_1

?oB_special:
	inc bx
	mov al, [bx]
	or al, al
	je ?oRet
	cmp al, 'A'
	je ?oB_bufA
	cmp al, '1'
	jc ?oB_dump
	cmp al, '9' + 1
	jnc ?oB_dump
		;; display the numbered argument #AL
	push bx
	cbw
	mov bx, ax
	mov bl, [bx+??strarg1-'1']
	call ?oString
?oB_1:
	pop bx
	jmp short ?oB_2
?oB_loop:
	cmp al, '%'
	je ?oB_special
?oB_dump:
	printALtoConsole
?oB_2:
	inc bx
?oBuffer:
	mov al, BYTE [BX]
	or al, al
	jnz ?oB_loop
?oRet:
	ret

?newline:
	;; Linefeed
	mov al, 10
	printALtoConsole
	mov al, 13
;;	printALtoConsole

;; Dump character AL onto screen
;; IN: AL = character

?oChar:
%if 0		;; no need when doing I/O through DOS
	push bx
	mov bx, [??0Earg]
	mov ah, 0eh		; teletype output
	int 10h
	pop bx
%else
	mov ah, 2		; output to STDOUT
	mov dl, al
	int 21h
%endif
	ret

%ifdef DEBUG
??_a DB "AX == 0x"
??_b DB 1,2,3,4, 13, 10, 0
??_d DB "Device name: >>"
??_c DB 1,2,3,4,5,6,7,8, '<<', 13, 10, 0
??dispAX:
	push bx
	push ax
	mov bx, ??_b
	call ??AXtoBX
	mov bx, ??_a
	call ?oBuffer
	pop ax
	pop bx
	ret
??AXtoBX:
	push ax
	mov al, ah
	call ??ALtoBX
	pop ax
??ALtoBX:
	push ax
	shr al, 4
	call ??NybbleToBX
	pop ax
??NybbleToBX:
	and al, 15
	add al, '0'
	cmp al, '0' + 10
	jc ??__c
	add al, 'A' - '0' - 10
??__c:
	mov BYTE [bx], al
	inc bx
	ret
%endif

%if 0		;; no need for I/O through DOS
??0Earg:
??actColour	DB 7
??actPage	DB 0
%endif

??repatchAddr DD 0

;?orgIOchannels DW 1234h

	;; Numerical arguments
??strarg1	DB 0					;; read/write
??strarg2	DB 0					;; area failed on
??strarg3	DB 0					;; error string
??strarg4	DB StrDelimiter			;; delimiter between two action strings
??strarg5	DB StrIgnore
??strarg6	DB StrRetry
??strarg7	DB StrAbort
??strarg8	DB StrFail

	;; alphabetical arguments
dummyByte:	;; This byte is destroyed, when no repeatCheck AutoFail is
			;; active for the drive
??strargA	DB '12345678', 0		;; drive letter or device name string

;; The repeat check is:
;; _+ activated if ?repCheck != -1
;;	+ the number of drives available to be checked is determined by
;;		counting the number of 0xFF bytes the immediately after the module
;;		got loaded into memory
%ifdef XMS_SWAP_CRITER
	global _criter_repeat_checkarea
_criter_repeat_checkarea:
%endif
?repCheck	DW -1	;; disabled
TIMES HIDE_CRITER_DRIVES DB -1		;; not displayed already

??allow:
??allowIgnore	DB 0
??allowRetry	DB 0
??allowAbort	DB 1	;; always allowed
??allowFail		DB 0
??input			DB '%4%6', 0		;; temporary storage


%ifndef COMPILE_STRINGS
??strings:
%else
%ifdef INCLUDE_STRINGS
%include "../strings/strings.err"
%else
??strings		DB 36	;; number of strings
	;; Here start all the strings!

DW S0
DW S1
DW S2
DW S3
DW S4
DW S5
DW S6
DW S7
DW S8
DW S9
DW S10
DW S11
DW S12
DW S13
DW S14
DW S15
DW S16
DW S17
DW S18
DW S19
DW S20
DW S21
DW S22
DW S23
DW S24
DW S25
DW S26
DW S27
DW S28
DW S29
DW S30
DW S31
DW S32
DW S33
DW S34
DW S35
DW S36

S0	DB 'reading from', 0
S1	DB 'writing to', 0
S2	DB 'Error %1 drive %A: %2 area: %3', 0
S3	DB 'Error %1 device %A: %3', 0
S4	DB 'DOS', 0
S5	DB 'FAT', 0
S6	DB 'root', 0
S7	DB 'data', 0
S8	DB '(I)gnore', 0
S9	DB '(R)etry', 0
S10	DB '(A)bort', 0
S11	DB '(F)ail', 0
S12	DB '? ', 0
S13	DB ', ', 0
S14	DB  8, 'IiRrAaFf', 0, 0, 1, 1, 2, 2, 3, 3

S15 DB 'write-protection violation attempted', 0
S16 DB 'unknown unit for driver', 0
S17 DB 'drive not ready', 0
S18 DB 'unknown command given to driver', 0
S19 DB 'data error (bad CRC)', 0
S20 DB 'bad device driver request structure length', 0
S21 DB 'seek error', 0
S22 DB 'unknown media type', 0
S23 DB 'sector not found', 0
S24 DB 'printer out of paper', 0
S25 DB 'write fault', 0
S26 DB 'read fault', 0
S27 DB 'general failure', 0
S28 DB '(DOS 3.0+) sharing violation', 0
S29 DB '(DOS 3.0+) lock violation', 0
S30 DB 'invalid disk change', 0
S31 DB '(DOS 3.0+) FCB unavailable', 0
S32 DB '(DOS 3.0+) sharing buffer overflow', 0
S33 DB '(DOS 4.0+) code page mismatch', 0
S34 DB '(DOS 4.0+) out of input', 0
S35 DB '(DOS 4.0+) insufficient disk space', 0
S36	DB 'Unknown error code', 0
%endif		; INCLUDE_STRINGS
%endif		; COMPILE_STRINGS
%endif		; AUTO_FILE


%ifndef NO_RESOURCE_BLOCK
%ifndef AUTO_FAIL
	DW ?repCheck
%endif
%endif

???ende:

%ifndef NO_RESOURCE_BLOCK
;; Include resource block
%ifdef AUTO_FAIL
	resIdBlock RES_ID_CRITER, 0
%else
%ifndef COMPILE_STRINGS
	resIdBlock RES_ID_CRITER, 1
%else
	resIdBlock RES_ID_CRITER, 3
%endif		; COMPILER_STRINGS
%endif		; AUTO_FAIL
%endif		; NO_RESOURCE_BLOCK

;;END

; Quote from RBIL
;--------D-24---------------------------------
;INT 24 C - DOS 1+ - CRITICAL ERROR HANDLER
;Notes:  invoked when a critical (usually hardware) error is encountered by DOS
;          (see #02543); should never be called directly
;        when DOS terminates a program, it copies the previous value of the
;          INT 24 vector out of the PSP (see #01378) and into the interrupt
;          vector table
;SeeAlso: INT 21/AH=95h
;
;(Table 02543)
;Values critical error handler is called with:
;        AH = type and processing flags (see #02544)
;        AL = drive number if AH bit 7 clear
;        BP:SI -> device driver header (see #01646 at INT 21/AH=52h)
;                (BP:[SI+4] bit 15 set if character device)
;        DI low byte contains error code if AH bit 7 set (see #02545)
;        STACK:  DWORD   return address for INT 24 call
;                WORD    flags pushed by INT 24
;                WORD    original AX on entry to INT 21
;                WORD    BX
;                WORD    CX
;                WORD    DX
;                WORD    SI
;                WORD    DI
;                WORD    BP
;                WORD    DS
;                WORD    ES
;                DWORD   return address for INT 21 call
;                WORD    flags pushed by INT 21
;Return: AL = action code (see #02546)
;        SS,SP,DS,ES,BX,CX,DX preserved
;Notes:  the only DOS calls the handler may make are INT 21/AH=01h-0Ch,30h,59h
;        if the handler returns to the application by popping the stack, DOS
;          will be in an unstable state until the first call with AH > 0Ch
;        for DOS 3.1+, IGNORE (AL=00h) is turned into FAIL (AL=03h) on network
;          critical errors
;        if IGNORE specified but not allowed, it is turned into FAIL
;        if RETRY specified but not allowed, it is turned into FAIL
;        if FAIL specified but not allowed, it is turned into ABORT
;        (DOS 3.0+) if a critical error occurs inside the critical error
;          handler, the DOS call is automatically failed (AL set to 03h and
;          the INT 24 call skipped)
;
;Bitfields for critical error type and processing flags:
;Bit(s)  Description     (Table 02544)
; 7      clear = disk I/O error
;        set   = -- if block device, bad FAT image in memory
;                -- if char device, error code in DI
; 6      unused
; 5      Ignore allowed (DOS 3.0+)
; 4      Retry allowed (DOS 3.0+)
; 3      Fail allowed (DOS 3.0+)
; 2-1    disk area of error
;        00 = DOS area   01 = FAT
;        10 = root dir   11 = data area
; 0      set if write, clear if read
;SeeAlso: #02545,#02546
;
;(Table 02545)
;Values for critical error code:
; 00h    write-protection violation attempted
; 01h    unknown unit for driver
; 02h    drive not ready
; 03h    unknown command given to driver
; 04h    data error (bad CRC)
; 05h    bad device driver request structure length
; 06h    seek error
; 07h    unknown media type
; 08h    sector not found
; 09h    printer out of paper
; 0Ah    write fault
; 0Bh    read fault
; 0Ch    general failure
; 0Dh    (DOS 3.0+) sharing violation
; 0Eh    (DOS 3.0+) lock violation
; 0Fh    invalid disk change
; 10h    (DOS 3.0+) FCB unavailable
; 11h    (DOS 3.0+) sharing buffer overflow
; 12h    (DOS 4.0+) code page mismatch
; 13h    (DOS 4.0+) out of input
; 14h    (DOS 4.0+) insufficient disk space
;SeeAlso: #02544,#02546
;
;(Table 02546)
;Values for critical error handler action code:
; 00h    ignore error and continue processing request
; 01h    retry operation
; 02h    terminate program as though INT 21/AH=4Ch called (INT 20h for DOS 1.x)
; 03h    fail system call in progress (DOS 3+)
;SeeAlso: #02544,#02546
;
;~
