[Coco] Re: Disk Basic and 512 byte block floppies.

[Gene Heskett]

On Sunday 18 January 2004 20:59, Theodore (Alex) Evans wrote:

>On Jan 18, 2004, at 2:58 PM, John E. Malmberg wrote:

>> Robert Gault wrote:

>>> tim lindner wrote:

>>>> John E. Malmberg <wb8tyw at qsl.net> wrote:

>>>>> I formatted a 720K 3.5 inch floppy with 512 byte sectors

>>>>

>>>> Interesting experiement.

>>>>

>>>> The size of each sector is written to the disk. Disk BASIC

>>>> expects all

>>>> the sectors to be 256 bytes, it doesn't check. It create 256

>>>> byte blocks

>>>> of RAM to read to and from these sectors.

>>>>

>>>> If the sector is larger, then the RAM buffer will overflow. On a

>>>> read sector command you will overwrite whatever is past the

>>>> sector buffer in

>>>> RAM. The could be dangerous.

>>>>

>>>> On a write sector command you will write whatever data is after

>>>> the buffer to the larger sector.

>>>

>>> With Disk Basic, the buffer overflow will go into a second disk

>>> I/O buffer; buffer0 $600-$6FF, buffer1 $700-$7FF. You would

>>> corrupt data stored in the second buffer but in most cases you

>>> won't see any ill effects.

>>

>> According to the Disk-Basic-Unraveled series, the buffer at

>> $700-$7ff is used for the verify function, and for the DSKINI

>> function.

>>

>> So with verify off, there should be no danger of data corruption

>> by using the DSKI$ and DSKO$ functions.

>

>I wouldn't use DSKI$ and DSKO$, I would use a call to DSKCON.

>Something like:

>

>10 DC=PEEK(&HC004)*256+PEEK(&HC005) 'DSKCON

>20 DO=PEEK(&HC006)*256+PEEK(&HC007) 'DCOPC

>30 CLEAR &H6800

>40 INPUT "SOURCE DISK",SD

>43 INPUT "SECTOR SIZE",SS

>47 IF SS=256 THEN SS=1 ELSE IF SS=512 THEN SS=2 ELSE GOTO 43

>50 INPUT "DESTINATION DISK",DD

>53 INPUT "SECTOR SIZE",DS

>57 IF SS=256 THEN DS=1 ELSE IF DS=512 THEN DS=2 ELSE GOTO 45


Shouldn't the above be THEN DS=2 ELSE GOTO 53?  There is no line 45


>60 POKE DO+5,0 'LSB OF BUFFER POINTER

>60 FOR T=0 TO 34

>70 POKE DO,2 'OPERATION CODE READ

>80 POKE DO+1,SD 'DRIVE NUMBER SD

>90 POKE DO+2,T 'TRACK NUMBER T

>90 FOR S=1 TO 18 STEP SS

>100 POKE DO+3,S 'SECTOR NUMBER S

>110 POKE DO+4,S+103 'MSB OF BUFFER POINTER

>120 EXEC DC 'CALL DSKCON

>130 NEXT S

>140 POKE DO,3 'OPERATION CODE WRITE

>150 POKE DO+1,DD 'DRIVE NUMBER DD

>160 FOR S=1 TO 18 STEP DS

>170 POKE DO+3,S 'SECTOR S

>180 POKE DO+4,S+103 'MSB OF SECTOR BUFFER

>190 EXEC DC 'CALL DSKCON

>200 NEXT S

>210 NEXT T

>220 END


-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty: soap,
ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
99.22% setiathome rank, not too shabby for a WV hillbilly
Yahoo.com attornies please note, additions to this message
by Gene Heskett are:
Copyright 2004 by Maurice Eugene Heskett, all rights reserved.