[Coco] Read coco disks on a PC

[John E. Malmberg]

On 3/15/2012 9:59 AM, Darren A wrote:
> On Thu, Mar 15, 2012 at 7:57 AM, John E. Malmberg wrote:
>>
>> While I have not had time to pursue this further, it means that the
>> following is doable:
>>
>> 1. A program could be written on a PC or any other platform that supports
>> 512 byte sector double density floppies to write the sectors on a double
>> density disk so that the COCO could read them.  As the number of sectors on
>> the disk get cut in half, it may have to mark them as unavailable in the
>> gran table.  The program also has to realize that the COCO Basic program
>> will only look at the first 256 bytes in the sector.
>>
>
> Not a good idea.  While the CoCo I/O routines will read 512 byte
> sectors, they do not ignore the last 256 bytes.  All 512 bytes will be
> read into memory even though the CoCo software has only allocated 256
> bytes for the buffer.

As I stated before:

The buffers are fixed in memory for Disk Basic.

The DISK IO routines will read all 512 bytes first into the disk buffer 
and then overrun into the verify buffer.

But when the ROM routines try to use the data, they will only read the 
first 256 bytes from the disk buffer.

A DISK basic program would have to use PEEK to read the upper 256 bytes 
or POKE to write to them.

The buffer overrun may cause problems with OS-9 or Flex-09 unless 
something is modified, but unless you have verify on, it is harmless to 
DISK Basic.

Regards,
-John
wb8tyw (at) qsl.network
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