Disk ROM? GROM?

[hippietrail]

When you upgraded your TI-99/4A from cassette tape to disk drive back in the day, I assume it came with an interface/adapter that plugged into the expansion port.

 

On many contemporary machines the ROM didn't have support for the DOS or the FDC and either the controller came with a ROM or code was loaded from the disk's boot block, or both.

 

Which did the TI-99/4A have? Of course it may have had a GROM rather than a ROM. Is there a disassembly available? Googling hasn't led me to the answer yet.

[HOME AUTOMATION]

TI-99/4A - Disk Memory System PHP 1240 multilingual manual

 

PHP 1240 Disk Memory System Manual

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Normally, I would scrounge the ROM from Classic99, by using MINI MEMORY, to switch CRU addresses.

Somehow, this looks wrong.

 

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Copped these from MAME:

 

fdc_dsr.u26

 

fdc_dsr.u27

 

[Tursi]

The TI has all device interface code in ROM - we call them DSRs "Device Service Routines". Every program that uses devices has code to search the ROMs for the one it wants.

 

Classic99's disk is a psuedo DSR - though you didn't look far enough to see where it ends. It starts with a completely valid DSR header that includes entry points for all the devices it supports (Classic99 supports both upper and lower case DSK names, and I believe CLIP and CLOCK are in the same DSR there). They jump to a 'magic' address that triggers the emulation to take over and parse the actual request.

 

Classic99 also does support the real TI controller, but bank switches it in automatically when a configured disk is accessed, so I don't think you can view it using mini memory. Same with the CF7 support.

 

[+FarmerPotato]

To be clear, every expansion device could contain its own DSR ROM. (Device Service Routine.) 
 

A utility routine like DSRLNK would search tables at the beginning of each ROM. These tables had a constant structure across cartridge ROM, GROM and expansion hardware.  This provided  compatibility between software (hopefully) and not-yet-invented hardware. 

 

A device ROM could provide several kinds of  table: power up initialization routine, programs for the  menu (sometimes), an interrupt handler. Table >8 defined the canonical names for devices like DSK1, with their handler for the common file opcodes, like OPEN, READ, CLOSE, etc. >A was weirder  and had  "subprograms" for lower level disk operations like FORMAT, READ SECTOR. 

 

Minicomputer Heritage

 

The DSR ideas in the 99/4 were borrowed from TI's minicomputer operating systems such as DX10, downsized as needed.  The key concepts were a standard pluggable table interface, shared file system concepts and common opcodes.*

This is not surprising, because TI employees in Home Computer software and hardware development would have used the 990 DX10 for their work.
 

 

* though DX10 required a new DSR to be linked into the operating system image by the sys admin. 
 

**TI-Writer and Hopper are known to have been written using a 99/4A. 

***There is sample DSR source in DX10 Vol 5, Systems Programmer Manual.