BBS: Inland Empire Archive
Date: 01-22-93 (11:14) Number: 369
From: CORIDON HENSHAW Refer#: NONE
To: MARK REJHON Recvd: NO
Subj: Heya. Conf: (2) Quik_Bas
Hello Mark!
In a msg of <20 Jan 93>, Mark Rejhon writes to Coridon Henshaw:
>> MR> The code needs a bit of optimization, though :)
>> Where?
MR> I'm an ASM programmer, and the conception of using strings to set the
MR> file attributes is _darned_ inefficient! :)
I designed those SUBs to be easy to use, not fast.
>> that the directory be opened as a file? You could use direct
>> disk access via int 13h to read the directory directly.
MR> I am capable of doing that, but where is the root directory,
Call int 21h with AH=1Fh to find it. This is best done in
ASM, because BASIC can't read unsigned values without extra
help. Here's code to do it:
GetDir PROC FAR USES DS, BX
;
;This is BASIC callable. It's also off the top of my head;
watch out for bugs ;
;
;Notes: Works ONLY in DOS 2 and 3. You'll probably want to
add version ;checking code. For this to work in DOS 4.0,
change "ADD BX,10h" to "ADD ;BX,11h"
;
;Works only on default drive. If you want to force a disk
to use, you have to ;change the default drive.
MOV AH,1Fh ;DOS function 1fh, get default disk parameter block
INT 21h ;Call DOS
TEST AL,FFh ;Was there an error?
JE Error ;If yes, jump to ERROR
;DS = segment of Drive Parameter block
;BX = offset
ADD BX,10h ;BX now points to root directory starting sector
MOV AX,[BX] ;AX = the starting sector of the root directory
RET
Error: MOV AX, FFFFh ;Return -1 to BASIC
RET
GetDir Endp
FUNCTION declaration for BASIC is:
DECLARE FUNCTION GetDir%
Now you have the sector of the root directory, next to
convert it into drive-head-track-sector notation for INT
13h. I don't know how to do this, so your on your own here. ]:>
MR> how do I read a whole subdirectory into a string (especially on a
MR> fragmented disk, as directories are simply special files, they
MR> can become fragmented too!)
Quite simply. First, find the location of the FAT. More ASM code:
GetFAT PROC FAR USES DS, BX
;
;This is BASIC callable. It's also off the top of my head;
watch out for bugs ;
;
;Works only on default drive. If you want to force a disk to use, you have
to ;change the default drive.
MOV AH,1Fh ;DOS function 1fh, get default disk parameter block
INT 21h ;Call DOS
TEST AL,FFh ;Was there an error?
JE Error ;If yes, jump to ERROR
;DS = segment of Drive Parameter block
;BX = offset
ADD BX,06h ;BX now points to root directory starting sector
MOV AX,[BX] ;AX = the starting sector of the FAT
RET
Error: MOV AX, FFFFh ;Return -1 to BASIC
RET
GetFAT Endp
BASIC calling syntax is:
DECLARE FUNTION GetFAT%
You've now got a disk pointer to the FAT. Next step is to
find out what kind of FAT it is. This is also something
that I don't know about. You'll have to ask someone else.
Once you have located the FAT, you need to convert sectors
into clusters. Every value in the FAT is in clusters, and
is in a linked list. Here's an example:
Directory entry points to cluster 00FCh, so you read the FAT entry for
cluster 00FCh, it points to the next cluster in the object,
read your data from that cluster, seek to the entry for
this cluster in the FAT, and so on. You've reached the end
of the chain when you read a 0FF8h-0FFFh entry (For a _12_
bit fat) or FFF8h-FFFFh entry (For a 16 bit fat)
You know how many bytes are in a object by it's directory entry.
MR> -After I read it and detect a directory in the data, where do I look
MR> for to go to that next subdirectory?
Scan every entry in the directory, and check it's DIR
attribute bit. Make sure to skip the "." entry, or you'll
have a endless loop.
Directory structure:
Offset Size Meaning
00h 8 Bytes File name
08h 3 Bytes File extiontion
0Bh 1 Byte Attributes, bit based
* 0 = Readonly
* 1 = Hidden
* 2 = System
* 3 = Volume Label (Root dir ONLY)
* 4 = Subdirectory
* 5 = Archive
* 6 = Reserved
* 7 = Reserved
0Ch 10 Bytes Reserved under MS-DOS, file
password under * DR-DOS
16h Unsigned Word Time of last update
18h Unsigned Word Date of last update
1Ah Unsigned Word Beginging disk cluster
1Ch Unsigned Dword Object size
MR> How do I account for fragmentation?
Via FAT reading
MR> -How do I know when I've reached the end of a directory?
When you hit a NULL filename.
I know this is quite a vage discription, but I'm getting
tired of typing now :))
\ Coridon Henshaw \ ³
\ Sirrus Software \ \ ³ /
\ Sysop, \ ÄÄÄÄ*ÄÄÄÄ
\ TCS Concordia \ / ³ \
\ 34:3416/18 \ ³
...I used to be sane, but now I'm better!
