Jump to content

Stuart

Members
  • Content Count

    1,022
  • Joined

  • Last visited

  • Days Won

    1

Stuart last won the day on September 18 2016

Stuart had the most liked content!

Community Reputation

886 Excellent

About Stuart

  • Rank
    Stargunner

Profile Information

  • Gender
    Male
  • Location
    Southampton, UK

Recent Profile Visitors

11,127 profile views
  1. 74LS362 is a correct designation for the clock chip on the earlier consoles, as Home Automation said above. What's the frequency of the crystal by the clock chip? If it is 48 MHz then the clock chip must be either a TIM9904 (no "A") or 74LS362 (they're the same). If it is 12 MHz then the clock chip must be a TIM9904A. (Wondering if it is a 12 MHz crystal but someone has fitted the clock chip for the 48 MHz crystal, so it is running 4 times slower than it should be with a flaky clock?)
  2. VDP and VRAM probably OK if no corruption if left on title screen. Ditto power supply. Can you type and run a simple "10 PRINT" in the few seconds available?
  3. Whether the colour bars are affected or not just depends on which particular memory bit is failing. Personally I'd start by replacing the VRAM - looks like the chip is failing as it gets warm. While you've got the console open you can clean up the VDP and apply some fresh heatsink goop.
  4. Aren't some parts of XB written in assembly?
  5. But of course changed to a 12 MHz crystal with the 9904A clock generator.
  6. Most 64K home computers of the period had BASIC written in assembly - usually around 16K - 24K. But they didn't have huge chunks of the memory map dedicated to the peripheral cards, cartridges and memory-mapped devices.
  7. I got the impression that the two sockets on the video card itself were for user programs - so the sockets are empty when you bought the card. Support for the video card is built into University BASIC. As MAME emulates that, MAME must include the ROM dumps? See lines 440 onwards of https://github.com/MisterTea/MAMEHub/blob/master/Sources/Emulator/src/mess/drivers/tm990189.c.
  8. "So do all the '|li' symbols refer to a virtual ground ..." Those symbols are the common ground on the PCB. The common pin on each regulator is connected to it to give a common ground point, as is one side of the bridge rectifier.
  9. I think the SID's *CS is tied permanently low - the SID is continually processing the data in the data latch that feeds it.
  10. (And further to Lee's explanation) the access time is significant for your (Vol's) previous argument that the 9995 is only faster if executing from internal memory. I couldn't find a copy of the Intel application note referenced but we could be fairly certain that the instructions are being executed from EPROM (otherwise why mention them?). We don't know though whether the data is processed in internal or external RAM on the 9995 - although it must of course be external RAM with the 9900.
  11. Extract from the TI Microsystems Designers Handbook, showing that the TMS9995 gives around twice the performance of the TMS9900, with the 9995 clocked from a 12 MHz crystal, and the 9900 with a 4-phase 3 MHz clock derived from a 12 MHz crystal.
  12. You still need to relate the measurements to Olympic-sized swimming pools and double-decker buses.
  13. Vol, have you read post #20 of
  14. I think you're getting confused with the clocks. With the TMS9900, you have a 12 MHz crystal feeding a TIM9904A which generates a 4-phase, 3 MHz clock for the processor. With the TMS9995, what they have done is incorporate the clock generator into the processor IC - internally the 9995 is still using the same 4-phase 3 MHz clock as the 9900. So you can't compare a [email protected] with a [email protected] - they're both internally using a 4-phase, 3 MHz clock from a 12 MHz timing source.
  15. "This is a very unusual processor. The external data bus is 8-bit." Not very unusual I think. The TMS9980/81 had the same - 16 bit internally, 8-bit external data bus. Intel done the same on some processors. "Instructions on the TMS9995 became much faster to execute, but only if they are located in the internal memory, or at least use data from the internal memory. But if an instruction and its operands are located in external memory, then it is executed, as a rule, even slightly slower. In addition, if we take an external clock frequency as the base, then even with the internal memory, the TMS9995 is slower than the TMS9900 at the same frequency." Only partially correct I think. The 9995 introduced instruction prefetch - it fetched the next instruction while processing the current instruction, and decoded the next instruction while storing the results of the current instruction. Much more efficient. Plus it didn't need the read-before-write cycle of the 9900. I'd be surprised if anything that the 9995 done was slower than the 9900, in internal memory or external.
×
×
  • Create New...