An EGA, MCGA or VGA can be detected via BIOS extensions and readable registers, particularly at 3C0-3CF. Hercules cards can be detected by some unused (by IBM) bits in the status port. A MDA adapter can be identified by reading from a status port at 3BA and a CGA by a status port at 3DA. Graphics cards can be detected by software. However, Tandy allowed its machines, starting with the SX, to use other graphics cards, complicating matters. graphics and sound are present in the machine. When the programmers have their programs search for these bytes, then they know that Tandy or PCjr. it is FD at F000:FFFE and 21 for the Tandy 1000s at FC00:0000. by reading identifier bytes in the BIOS area of these machines. They can detect a Tandy 1000 or an IBM PCjr. Games cannot detect the Tandy sound chip because it can be written to but not read. While this was not the only reason why the sound chip was not generally available, it was a daunting hurdle when every game that used it had to write directly to the registers located at the contested I/O location(s). Although the 2nd DMA controller was not really important until the mid 90s, PC clones that wanted to manufacture AT compatibles had to include it. The blame for this was on IBM, which had designed both the PCjr. The main reason why no one put the 3-voice TI SN76496 sound chip inside their PCs was because it utilized the same address space (C0-C7) as the 2nd DMA controller in the IBM PC AT and clones. They were not generally available in PCs and never available to the general public as a sound card. However, Tandy sound chips were almost exclusively found in Tandy 1000s and PCjrs. As far as sound went, the Tandy 3-voice chip was the only alternative to the PC Speaker. LucasArts' early SCUMM engine games also supported Tandy graphics and sound. Sierra's AGI engine games, which formed the bulk of their mid 80s PC offerings, supported Tandy graphics and sound.
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