News | Documentation | Hardware | Software | Books | Links

Direct Sector Access using

Joel Dinda [75725,1134]

While FLOPPY isn't my favorite program, it provides TDD2 users the capability to bypass the TDD file structure and access disk sectors directly. The most obvious applications for this capability are utility programs (there are several such in this Library) and (random access?) database programs (aside: database programmers will have to write their own file-management routines, a matter this discussion ignores). This file lists all-in-one-place those things someone writing such a program will need to know.

First I'll talk about syntax, then I'll discuss some necessary concerns, finally I'll discuss some background matters.

WARNING: This is an advanced and fairly specialized programming tip. Only fairly experienced M100(etc) programmers will understand some of this discussion. Lots of it is pretty opaque. While it's not intended to intimidate, you've got to be writing a pretty sophisticated program to want to know these things.

Powr-DOS/TDD1 users/programmers will want to read DOSTIP.009.


The necessary command is a conventional BASIC CALL:



 Z is the start of FLOPPY's file transfer routine (depends upon which computer you're using)  Y is the code for the activity you wish to provoke (details follow).  X is the RAM address of some further instructions necessary for execution of the routine.

 Z options:

 In M100/T102, Z is 62297 (-3239 integer).
 In T200, Z is 58980 (-6556).

 Y options:

 1 attempts to format the diskette.
 3 attempts to read the diskette.
 4 attempts to write to the diskette.
 5 checks (reports) the status of the drive (so do the other options).

 D2TIP.005 says, incorrectly, the Y=2 writes to the diskette (easy mistake). If someone figures out what Y=2 actually *does* do, I'd like to hear about it. Thanks.


 X options:

 Any legal RAM address--but some are far better than others. I'll return to this momentarily.
Further information at location X:

The routine beginning at Z checks at X for two pieces of information:

X and X+1 store the address, in RAM, reserved (by you) for the file transfer.

(Call it a buffer [BF]; this is its lowest address [possibly HIMEM]. I discuss this further below.) Like most M100 two-byte addresses, these are stored LSB+MSB. (I suggest a simple solution in the discussion below.)

X+2 is the diskette sector being transferred.

It may help to think of the information at X this way: You're transferring data [content and direction unspecified] between a buffer which is indicated at X & X+1 and a diskette sector which is indicated at X+2.


Necessary Concerns:

M100 or T200?

By this time there's no excuse for donating a computer- specific program to this SIG without good reason. PEEK(1) returns 51 in M100/T102; it returns 171 in T200. Once you know this, you can assign the appropriate value to Z and any other computer-related POKEs, PEEKs, or CALLs you'll be using. (I like to work over the keyboard buffer, for instance.)

Is FLOPPY there?

Next, check to see if FLOPPY's actually installed. Tandy's BACKUP does this by checking four RAM locations; the following line is a slightly more compact version of theirs:

IF PEEK(Z)=229 AND PEEK(Z+1)=94 AND PEEK(Z+239)=4 AND PEEK(Z+240)=195
THEN continue ELSE error: "No System"


Next create one or more buffers for your program's use.

You'll need one buffer for each sector you intend to duplicate in RAM at any one time; the exact number will depend either upon the nature of your application or the amount of RAM available. Each buffer will be 1284 bytes long; it should be locked in with some variation of the following instruction:

CLEAR256,HIMEM-(1284*(number of buffers))

It is always good practice to restore HIMEM to its previous value when the program finishes; there are at least two workable schemes. Folks who use MAXRAM instead of HIMEM in programs which overwrite high memory are despicable creatures; they should, at least, warn users that they're destroying files.

Variable X:

The third CALL argument points to a RAM address containing address information for the CALL's use. I know of two ways to do this, but suggest that everyone use this one:

If BF is the first (lowest) address in the buffer, and S is the sector number:

0 ... :DEFINT T:DIM T(1):

16 ... :T(0)=BF:T(1)=S:CALL Z,Y,VARPTR(T(0)): ...

This is a "cute trick"; it works because M100 stores T(0) and T(1) together in RAM; VARPTR(T(0)) tells that location. [Please notice the DEFINT! It matters!]

Some of the SIG's utilities use POKEs to accomplish this, but there's no advantage (and the coding's more difficult).

Detecting Errors

While this CALL does not directly return error messages, it does return error information. This information is stored at address X; if you've used VARPTR(T(0)), you only need to determine the value of T(0). Good practice (and common sense) dictate that you check this value immediately after attempting any file transfer.

These are the documented errors:

T(0)=0 or 1 no error
T(0)=3 RS232 Not Ready (Powr-DOS Error 59/NR)
T(0)=5 Communications Error (61/CM)
T(0)=6 Write Protected (63/WP)
T(0)=8 No Disk in Drive (65/ND)
T(0)=9 Hard Trouble (66/HT)
T(0)=12 Drive Not Responding (60/DN) [not previously documented]

Presumably the "empty" numbers can be generated, but I haven't figured out how. If you discover one, please pass it along. Thanks.

I've been using the following approach. Obviously there are others, but I'm happy with this one:

0 ON ERROR GOTO 99: ...
16 ... :CALL Z,X,VARPTR(T(0)):GOSUB98: ...
98 IF T(0)<2 THEN RETURN ELSE IF T(0)=6 OR T(0)=8 THEN ERROR 63 ELSE IF T(0)=3 OR T(0)=5 OR T(0)=9 OR T(0)=12 THEN ERROR 60 ELSE ERROR 99
99 IF ERR=63 THEN PRINT "Disk Error" ELSE IF ERR=60 THEN PRINT "Drive Error" ELSE PRINT "Error" ERR "in line" ERL:END

I've used ERROR 99 as a clue that we've found a new error code. The other codes simplify conversion between Powr-DOS & FLOPPY.

You can modify this to RESUME or quit, as appropriate. For instance, knowledge that T(0)=6 means the diskette is write protected can be used with CALL Z,5,X force the user to protect a diskette's files.


Much of the technical information was obtained by studying Tandy's BACKUP.BA, which is supplied with the TDD2 drive, and by systematic experimentation. Some of the other discussion follows from Powr-DOS experience.

Buffer Format:

The first byte in the sector (more exactly, in the buffer) (let's call it BF+0) indicates the diskette format in use. 0 means it's a TDD1 diskette; 22 means it's a TDD2 diskette. This is evidently provided for our information, as changing it has no effect--if you "save" a sector with this byte changed, then retrieve it again, it's been restored to its original value.

The second byte in the sector (well, buffer) (BF+1) is the file vector:

Since file deletions do not modify these vectors, these may be misleading.

Two bytes of unknown consequence follow at BF+2 & BF+3. They appear to always be zero; presumably they could contain information with meaning to the drive.

The next 1280 bytes (starting at BF+4) duplicate the sector's contents. If the sector is not 0 or 1, this could be anything (TDD2 doesn't inflict any format). Unless modified, Sectors 0 and 1 conform, with three significant modifications, to the information in SECTR0.TDD, available from this Library. Discussion follows immediately....

Directory Structure:

TDD directory structure is discussed at some length in my file SECTR0.TDD. Since FLOPPY delivers a somewhat different copy of the sector to RAM, a few adjustments must be made; some of these adjustments result from differences between the drives, while others seem to be idiosyncratic programming decisions.

Perhaps the most important difference between sector buffers created by Powr-DOS and FLOPPY is that FLOPPY's sectors come with four leading bytes (two of which contain information), while P-DOS's come with 12 trailing bytes (one of which contains information). When reading SECTR0.TDD, you must therefore add an offset of 4 to most counts, while the file vector check must be adjusted from BF+1281 to BF+1.

Since TDD2 diskettes contain two directories (Sectors 0 and 1), certain allowances must be made for this. Except for the allocation table, the structure of the directory sector(s) is unchanged from that described in SECTR0.

The allocation table is changed. Where TDD1 stores allocation information for 80 sectors on half of 160 bits, TDD2 (naturally) uses all 160 bits to store twice as much information. The allocation table is duplicated on both directories, effectively describing the entire diskette for either.

Important related matter: FLOPPY (and any other DOS, near as I can tell) updates both allocation tables when you add or subtract a file. One side effect of this is that Sector 1 should be considered unavailable under all circumstances--unless you're writing your own file control routines and never use the TDD2's built-in routines for file transfers.

Enough. That should point you in the right direction....
Joel Dinda
July 4, 88

ureturn.gif (2080 bytes)