Because the old gear and the old diskettes are so cranky and/or fragile, and in the case of some of the components, so hard to find or expensive, many old TI users have turned to simulated environments that run on their everyday PCs. To some degree or another, these environments tend to use actual TI code (emulators) which run against an emulated TMS9900 processor and other components, or regular PC code (simulators) which imitates the be behavior of the TI without the representation of all the internal memory, hardware lines, etc.
None of these environments can read or write to a TI diskette directly, because of key differences between the architecture of a TI diskette and a typical PC diskette. So all of the environments rely on either individual images files which represent TI disk files, or more commonly, larger files which are binary image of a TI diskette.
The disclaimer here is that I only know about the ones I've used, and my exposure is a general user's exposure. These are my personal observations and not a definitive review of the whole scene or even of any one environment. On the other hand, since most of the emulators/simulators are free software (with one significant exception) I've got at least a little exposure to almost all of them.
V9T9 was one of the first DOS based emulators. It also benefitted from a third-party front-end to hide its complexity. Alas, neither V9T9 or the best front-end have been supported for a number of years. Nevertheless, the V9T9 diskette image format persists as the most common, perhaps because it's the most simple: a sector-by-sector and byte-by-byte binary image of the TI diskette.
Just the FAQs: "MESS is an acronym that stands for Multi Emulator Super System. MESS will more or less faithfully reproduce computer and console systems on a PC. MESS can currently emulate over 250 systems from the last 5 decades.
"MESS emulates the hardware of the systems and sometimes utilizes ROM images to load programs and games. Therefore, these systems are NOT simulations, but the actual emulations of the hardware."
Well, OK. MESS came from MAME, which was an emulator for arcade machines. It has branched out.
The complete MESS package is pretty nifty, but people who actually use it usually run it through a front-end to control the options and configuration. Yes, it's complicated. For the Ti-99, there are usually 'issues' that pop up with each new build. I have a CD which was distributed by one of the members of the Yahoo group mailing list, and it contains a full 'stable' version, a front-end, and it works pretty well, at least when emulating a Geneve with a hard drive.
The users who emulate the Geneve. and/or various flavors of hard drive, tend to think that this one is the best, because it handles those environments the best.
PC99 is commercial software sold by Cadd Electronics. To quote their website: "PC99 is a software-only package that emulates the Texas Instruments TI-99/4A Home Computer and selected TI and third-party peripherals." PC99 is DOS based, but will run in earlier Windows versions that support true MSDOS sessions.
PC99 uses licensed versions of actual TI GROMS ('GROM' was a proprietary TI ROM with auto incrementing addresses) but only comes with a few essential cartridge modules. The others are 'for sale separately.' I'm often tempted to buy a copy, if only to gain access to the wonderful CD called The Cyc containing tons of PDF files, diskette images, source code, and other TI goodies. PC99 does not emulate the CorComp disk controller, however, or the Gram Kracker, two of my favorite peripherals.
PC99 users a proprietary format for its diskette images, and includes a set of utilities for transferring files and disk images between a PC and a real TI system. As of this writing I do not own PC99 and have not decoded its diskette image format.
Classic99 is a good emulator written by a good guy, but it doesn't use diskette images. Instead it has you populate directories with TI programs and data, and accesses those directories as TI diskettes. The only thing wrong with that scheme is that you loose the characteristics of the diskette itself: no disk name, no header, no sector map. So if your software wants to use any of that, it breaks.
Classic99 includes a debugger.
Win994a by Cory Burr is one of the most complete simulators. It handles V9T9 format diskette images (although in insists that they have a .TIDisk extension) and it actually produces speech through its simulated speech synthesizer. It includes a a separate program which provides a complete environment for writing and assembling programs in TMS9900 assembly language, and it includes a very nice diskette (image) manager.
It's pretty astonishing, really, but it is limited in its ability to simulate third-party hardware, like the Gram-Kracker, for example.
Jaime Malilong's device is hardware based, but it does, sort of, emulate pretty much everything except the console and display. Furthermore, like emulators and simulators, it allows one to use binary diskette images instead of those pesky floppies. In fact, it forces one to use images, since the device is incompatible with any physical diskette controller.
To use floppy images, they must be transferred to a Compact Flash card using Jaime's supplied software. Jaime also provides software to transfer the images back off of the CF card, and has recently added a TI program which acts as an image manager. The CF card is then inserted into the device for use with a TI console.
Within the hardware device, Jaime manages to add some BASIC utilities for MOUNTing and UNMOUNTING an image onto one of three simulated drives which can then be accessed in the normal fashion. With the latest version he also emulates the TI 32Kb memory expansion and provides a parallel port.
The device comes complete, but a bit ungainly as it is without a case. It plugs into the console's expansion port, and draws its power from there. It can be daisy chained with a speech synthesizer, but in that case the power needs to come from a separate low-voltage power supply.
Jaime's software can read and write V9T9 format diskette images, but images created using the CFMGR program have a non-standard which is incomplete except for binary flags that signify a SSSD diskette with 1600 sectors. If you 'reformat' the new floppy image with a 'standard' tool like TI Disk Manager or DM1000, the image header is changed back to the full standard TI format.
When any image is pulled from the CF card by Jaimie's utility it's size is 400Kb, enough to hold Jaime's default of 1600 sectors, even if it's an image of a 90Kb diskette. Obviously, his software essentially ignores header information like number of sides and density.
Nevertheless, the images seem to be OK to use with other software-based emulators.
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