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So assuming one wanted to buy a used one of these (I had timex sinclairs around this time) how would one display the composite video nowdays?
@UsagiElectric on YouTube has a series of videos on building a homebrew around the TMS9900 processor. Would be cool if a unix-like OS could be used on something like that, though sounds like this project is specifically targeting the TI-99/4A system. The TI-99/4A was the first computer I owned as a teenager. I had used TRS-80s and Apple ][ at school. I eventually bought the expansion box and a couple of accessory car…
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Yep, but it lacks a MMU so memory protection and paging are going to require a lot of work. I think the only reason this is feasible at all is they're running the OS out of a ROM cartridge.
Did the minicomputers of the time have MMUs? I thought UsagiElectric showed a case where his Centurion didn’t, but I may be misremembering.
The 9900 is a single chip implementation of the CPU board in the TI 990. They even created a dedicated memory mapper chip to go along with the product line, though it is significantly different than the one in the minicomputer line.
(edit: the 990 was first built in the early 70s, memory mappers are quite old conceptually)
(edit 2: in fact the necessity of using a memory mapper is what killed the platform, and was one of the things that made the IBM PC team decline the 9900.)
Unfortunately I don't think that there is a reasonable way to perform real hardware-level memory protection with that chip alone. I'm working on a project documenting the genesis of the 99000 chips, which include a privilege bit in the status register, from the minicomputer line.
Essay forthcoming, and probably an OS. Maybe a year...
Earlier quoted context omitted.
Yep, but it lacks a MMU so memory protection and paging are going to require a lot of work. I think the only reason this is feasible at all is they're running the OS out of a ROM cartridge.
The PDP machines that Unix was developed on had MMUs, which they needed because the 16 bit processors couldn't address the multi-megabyte address space the hardware supported. I'm pretty sure the Centurion doesn't run Unix.
@UsagiElectric on YouTube has a series of videos on building a homebrew around the TMS9900 processor. Would be cool if a unix-like OS could be used on something like that, though sounds like this project is specifically targeting the TI-99/4A system. The TI-99/4A was the first computer I owned as a teenager. I had used TRS-80s and Apple ][ at school. I eventually bought the expansion box and a couple of accessory car…
Earlier quoted context omitted.
Did the minicomputers of the time have MMUs? I thought UsagiElectric showed a case where his Centurion didn’t, but I may be misremembering.
In fact the 9900 itself was used in an entire line of minicomputers that included a hardware memory map. The 9900 is a single chip implementation of the CPU board in the TI 990. They even created a dedicated memory mapper chip to go along with the product line, though it is significantly different than the one in the minicomputer line. (edit: the 990 was first built in the early 70s, memory mappers are quite old conc…
Earlier quoted context omitted.
While the CPU is a better fit than the 8 bit contemporaries, the 16kb of working memory is going to be a struggle.
It's cool because the registers are all in RAM, with a "workspace pointer" on the CPU pointing at where they are. This is slow, but a context switch is just changing that pointer.
The 6502 is not very different with a very small number of registers and Zero Page being used for most of what a modern machine would use registers for. For example (unlike the Z80) there is no register-to-register add or subtract or compare -- you can only add/sub/cmp/and/or/xor a memory location to the accumulator. Also, pointers can only be done using a pair of adjacent Zero Page locations.
As long as you were using data in those in-RAM registers the TI-99/4 was around four times faster than a 1 MHz 6502 for 16 bit arithmetic -- and with a single 2-byte instruction doing what needed 7 instructions and 13 bytes of code on 6502 -- and it was also twice as fast on 8 bit arithmetic.
It was just the cheap-ass main memory (and I/O) implementation that crippled it.
@UsagiElectric on YouTube has a series of videos on building a homebrew around the TMS9900 processor. Would be cool if a unix-like OS could be used on something like that, though sounds like this project is specifically targeting the TI-99/4A system. The TI-99/4A was the first computer I owned as a teenager. I had used TRS-80s and Apple ][ at school. I eventually bought the expansion box and a couple of accessory car…
So assuming one wanted to buy a used one of these (I had timex sinclairs around this time) how would one display the composite video nowdays?
A USB video capture device or a converter box. There are devices sold specifically to interface these old machines with modern displays. One of the more famous ones is the RetroTINK.