UPDATE TO SEEKRATE CHANGES FOR USING 5 1/4 " DISK DRIVE

@atari

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atari

· 4 years ago

This is in reference to Dave Small's article on adding a 5 1/4 " IBM PC compatible disk drive to the 520ST.

When I connected a 40 track double sided 5 1/4 " drive as drive B to my system I found I could read the directory okay but I could not read any files that were more than one or two tracks from track 0. The track step-rate or 'Seekrate' used for the ATARI drives was to fast for the slower 5 1/4" PC compatible drives.

The default 'seekrate' used on bootup in the 520ST is 3 ms.. The 5 1/4" drive I used requires a 6 ms. step-rate. The published memory location for the default seekrate is $400. However it does no good to change the value in this location after power up because it is only used in the boot process. All subsequent disk I/O uses a parameter called 'dseekrate' in the BIOS listings. The value for 'dseekrate' is stored in the 'DSB' for the specific drive. Part of the initialization process in the boot sequence is to move the default value in $400 to the 'dseekrate' location in the 'DSB' for each active drive.

The problem was finding the location of the 'DSB' and the corresponding byte for the appropriate 'dseekrate'. Dave Small had a message on this board last week giving the location of $6CB. This is location of the 'dseekrate' for drive A on a system operating with TOS in RAM. Location $6CF is the corresponding location for drive B. These locations are not the same if you are fortunate enough to have TOS in EPROMS.

The following are the addresses for the 'dseekrate' parameters that I have found appropriate for my system using either RAM TOS or ROM TOS.

            ORIGINAL TOS on DISK            TOS in EPROM 
---------------------------------------------------------------------------- 
Drive A            $6CB                         $A09 
Drive B            $6CF                         $A0D

The default values in these locations will be 03. For 6 ms step-rate the value should be changed to 00. The following is a list of all valid values;

    Step-Rate         Value 
------------------------------- 
      6 ms            $00 
     12 ms            $01 
      2 ms            $02 
      3 ms            $03

After booting the system up with the 5 1/4" drive active, change the 'dseekrate' location appropriate for your configuration using either 'SID' or a memory editor utility such as is available with the Holms & Duckworth ToolBox or the HIPPO Disk Utilities. You should be able to accomplish the same thing from BASIC using 'POKE' if you subtract one from the byte address since BASIC will poke a 'word'. For example from BASIC use $6CE for location $6CF.

I have used 6 ms ($00) with my 5 1/4" drive and it works flawlessly.
I format my disks on an IBM compatible system. All other operations, reading, writing, copying, deleting, etc., work the same as the 3 1/2" drive. The real advantage, is being able to load applications such as software for LOTUS 123 from an IBM compatible disk and run it under VIP.

James H. Trageser San Diego, CA