Re: Tips for making Twiggy images

From: Shirl Casner <shirlgato_at_email.domain.hidden>
Date: Thu, 5 Feb 2015 12:56:49 -0700

Hi James,

Thanks very much for the Lisa Twiggy and Basic Lisa Utility (BLU) disk utility info, always appreciated. I also had a chance to look at the BLU manual and this seems like a very impressive utility. Thanks for making this.

> 12 is the number of tag bytes supported by
> the Lisa Floppy Disk Controller

This is surprising. From the Lisa hardware manual and the device driver manual, I thought the tag size was 24 bytes. The tag record definition in the driver manual occupiers 24 bytes. The last 12 bytes of this definition seem extra in that the first 12 bytes which includes the FILE ID and ABSOLUTE PAGE number should be sufficient to reconstruct a disk whose directory was damaged.

I rechecked the Lisa hardware manual (1983 version, pages 6-5b, 6-32a) and see that the buffer assigned for the Twiggy disk block storage (called I/O BUFFER) is 524 bytes which means this holds 512 bytes of block data and 12 bytes extra. I also looked at the Lisa I/O ROM firmware listing (http://bitsavers.trailing-edge.com/pdf/apple/lisa/firmware/IO_ROM_88_Listing_Dec83.pdf), it too shows 12 bytes for the BLOCK HEADER (page 4).

Seems the Lisa device driver manual and the Lisa OS manual are wrong and James is right again !

I learn something everyday about the Lisa even after 32 years :-)

Regards,
David Craig

===

On February 5, 2015, at 12:28 AM, James MacPhail wrote:

>

>> If you do produce an image of these Workshop Twiggy disks, please
>> include the TAG DATA too. I don't know how BLU works, so don't know
>> if including tag data is the default.

>
> BLU always includes the tags in the DC42 image when a twiggy is read.
>
> (BTW, I vaguely recall that it will accept a DC42 image with no tag bytes when writing a disk from an image, as a few Mac images like that were found.)
>

>> Per the Macintosh Disk Copy v4.2 format

>
> Note that the twiggy image created by BLU is in DC42 format as much as possible, but DiskCopy itself (and probably all other software, with the possible exception of a Lisa 1 emulator) has no knowledge of twiggy disk images and won't be able to do anything with it.
>

>> 12 (the tag size the BLU utility seems to use)

>
> 12 is the number of tag bytes supported by the Lisa Floppy Disk Controller. Since the CPU is well insulated from the disk hardware by the controller, it would be quite difficult to implement more tag bytes.
>
> James

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Received on 2015-02-05 15:00:23