Friday, August 29, 2014

Progress report for August

Apologies for being quiet for a while - lots of tuning and small changes have been taking place. It's probably easiest to report just what's been going on by sharing this excerpt from the changelog:

ZX Prism version 0.12
Archived: 25/8/2014 (non-released, but flashed to board)

  • After noting that shadow warriors screen corruption stopped in 7MHz mode (though it still crashed), added check for CLK_EN='0' for SRAM_WE. This stopped the screen corruption in all CPU speeds though the game still crashes.
  • added clock enable check for port 7FFD
  • Altered CPU speed selection to give choice of spectrum clock derived CPU speeds or Video clock derived speeds: 3.18MHz, 3.5MHz, 6.375MHz, 7MHz, 12.75MHz, 14MHz, 25.5MHz, 28MHz and 51MHz
  • Altered interupt generator so it generates a CPU or Video clock derived interrupt.
  • Fixed interrupt frequencies (CPU speed derived interrupt was being generated at 100Hz  not 50Hz!
  • Fixed odd crashes at higher speeds... Now works up to 51MHz. 
  • Disabled 56MHz clock speed as I couldn't get it to run without random crashing
  • Reinstated partial address decoding for port 7FFD (fixes crashes/odd behavior in some 128K games which don't fully address the port). Am currently using the partial decoding as used on the Spectrum +3, +2A and +2B
  • Fixed another problem with interrupts - the video clock derived interrupts were running at half speed.
  • "tuned" interrupt signal duration so that they are different for each clock speed, using R-Type as a test. Settled now for values for each clock speed which i) don't cause R-Type to crash, ii) Allow R-Type to detect keypresses, iii) Allow R-Types screen updating to happen properly. These will not likely be the final durations ;)
  • Made video domain interrupt calculator equal frame length equal to 1 VGA frame (oops)
  • Successfully loads Speedlocked game (Sabre Wulf from They Sold a Million) at double speed (with CPU at 7MHz) and quad speed (with CPU at 14MHz) from audio CD

  • Flashed this version to FPGA board via Active Serial (so that it runs as soon as the board is turned on instead of having to be configured by the PC each time)
Loading a ULAplus enhanced game

And another ULAplus enhanced game. Hurrah for The Mojon Twins!

ZX Prism version 0.11
Archived: 18/8/2014 (non-released)

  • Added support to read palette entries from ULAplus (IN 65339)
  • Altered Mike Stirling's keyboard driver to handle cases where multiple keyboard rows are scanned by software (fixes "press any key" problem in Pssst etc etc etc)
  • Fixed bus timing issues introduced/uncovered by changing keyboard driver (!)
  • CPU speed stable to 28MHz
  • Fully implemented rest of port 7FFD including latch locking for ports 7FFD and 1FFD
  • Small update to VIDEO driver to allow screen selection
  • Implemented scanning of function keys. F12 sends a reset signal to CPU etc. F1-F11 send signals to the "buswatch" glue logic block to act as NMI keys, turbo buttons etc.
  • Implemented F12 = reset
  • Implemented F9 = Unlock ports 7FFD/1FFD if locked
  • Implemented F5 - F8 as CPU speed selectors (allows me to test games at different CPU speeds without having to hack their loaders which was getting tedious..)
  • Made keyboard scanning synchronous with CPU at whatever speed (stops Starstrike II crashing)
  • implemented F1-F4 as NMI keys (all 4 just send NMI to CPU at this stage)
  • fixed VRAM timing error which resulted in noise on screen on some games (eg Gyroscope)

ZX Prism version 0.1
Archived: 12/8/2014 (non-released)


  • Spectrum ROM implemented as on-chip memory, hard-coded into 0x0000
  • Implemented extra sceen resolutions (512x192, 256x384, 512x384)
  • Implemented screen decode methods: Standard, 16+16, 32 colour, 256 colour 1, 4 plane planar
  • Implemented SRAM for 512K memory pagable into 0xC000; page 2 hard-coded into 0x8000
  • VRAM page 0 hard-coded into 0x4000, no shadow screen selection yet
  • Port 7FFD partially implemented (memory select on bits 0-2, 6 and 7)
  • CPU speed selectable. Tested at 3.5Mhz and 7MHz
  • Default palette defined (except last 8 entries!)
  • ULA2 palette implemented but colour definition currently broken
  • Basic ULAplus implementation (can turn on and off and redefine RGB palette)
  • PS/2 keycode -> spectrum keyboard scanning implemented (based on Mike Stirling's FPGA Spectrum keyboard code)


  • Simple kempston joystick using on-devboard keys
  • Reset key implemented
  • PS/2 re-implemented off-devboard due to hardware issue and 2 dead FPGA pins
  • VGA re-implemented off-devboard to achieve 16 levels of red, green and blue (on-board VGA is 8 colour total)
  • Current RAM and ROM page displayed on 7-seg LED array
  • Tape IN and Tape OUT implemented via MX3232 RS232 UART.
  • Tape interface thoroughly tested and tweaked
  • Tape OUT tested with headphones only, haven't tried to SAVE
  • Beeper output directed to TAPE OUT for now (beeper port of ULA is XORed with the tape out port - some music routines, eg Manic Miner, don't work properly)

Software tested against v0.1 (and subsequently used to test fixes in later revisions):

  • Aquaplane (as expected the border effect doesn't work due to VGA timings and lack of contended memory)
  • Manic Miner (music didn't work due to how beeper is mixed with tape-out; ie digitally rather than in analog circuit)
  • Myth (seemed to work fine)
  • PSSST! (from ROM) (worked fine except 'press any key' only accepted keypress from a single row)
  • Fred (worked fine)
  • Kokotoni Wilf (graphical corruption; this may have been a dodgy load, must verify)
  • Gilligan's Gold (worked fine)
  • The Pyramid (worked fine)
  • Starbike (worked fine)
  • Match Point (worked fine)
  • Freddy Hardest (worked fine)
  • Buck Rogers (worked fine)
  • Midnight Resistance (loaded fine, redefine keys option didn't work)
  • Skool Daze (worked fine)
  • Slap Fight (worked fine though Slap Fight logo appeared to have some minor corruption -load error?)
  • Saboteur (worked fine)
  • Horace Goes Skiing (worked fine, seemed a little fast though - lack of contention?)
  • Bionic Commando 128 (crashes after loading first block on tape)
  • Super Sprint (worked fine except border effect didn't work, as expected)
  • Where Time Stood Still (worked fine)
  • Sabre Wulf (alkatraz loader from Ultimate Collected Works) (worked fine)
  • Uridium Plus (seemed to be working but couldn't move ship up and down - fire and reverse direction worked)
  • - Shadow Warriors (graphical corruption then crash)

ZX Prism loading Ghouls N Ghosts from (audio) CD

Saturday, August 9, 2014

Small update

Just a small update for today as it's been a fairly busy week so I've not had much time to spend working on ZX Prism lately.

Firstly, ZXPrism now has 512K available to it (in addition to the 32K video RAM). Up until this week, ZX Prism was running as a 32K machine, with its video RAM paged into the memory space 0x4000 - 0xBFFF. This arrangement was merely for testing. Prism's memory arrangement is now much more like you'd expect on a Spectrum 128, with video RAM at 0x4000 - 0x7FFF and 'ordinary' RAM between 0x8000 and 0xFFFF.

Like on the Spectrum 128 etc, the extra RAM is split into 16K banks which get paged into the area 0xC000 - 0xFFFF; as ZX Prism has 512K, this equals 32 x 16K pages of RAM which I'll refer to as pages 0 - 31. Page 2 is always paged into memory area 0x8000 - 0xBFFF (currently; this page may be changed to be selectable in future). The memory which is paged into 0xC000 - 0xFFFF is selected using IO port 7FFD - the same as on the Spectrum 128 - but of course ZX Prism has more pages available than on the 128 so ZX Prism uses bits 6 and 7 of port 7FFD in addition to bits 0 to 2. This matches the way that the Russian Pentagon 512 does it:


D0 - page[0]
D1 - page[1]
D2 - page[2]
D3 - display screen[bit 1] (msb - ZX Prism has 3 shadow screens, not 1)
D4 - ROM bank select[bit 0] (Prism will eventually have 512K flash ROM)
D5 - Prevent further memory paging (also known as "48K mode")
D6 - page[3]
D7 - page[4]

ZX Prism's SRAM is actually 256K of 16 bit RAM. Bit 4 of the page number determines if the upper or lower byte of the 16 bit data bus are to be used, effectively splitting the SRAM into two lots of 256K which is how 512K is achieved.

Currently ZX Prism's ROM is stored on the FPGA and is the original Sinclair ROM from the 16/48K Spectrum. As such, the ROM bank select currently does nothing. The "lock" to 48K mode also isn't yet implemented (though it would be trivial to do so), so ZX Prism currently perpetually runs in "USR 0" mode.

The additional memory was tested a number of ways - firstly RAMTOP was set to below 0xC000 and then unique test data was written to each page; then the data from each page was read back and verified. Of course there's not much 512K software (at least not that's available as a TAP/TZX file) to test, but I've tested several 128K games.

Speaking of tapes, I'd been finding loading to be a little unreliable. Everything would be OK for 3 or so minutes but then it would fail. It didn't matter if this was a turbo loader or standard loader. Eventually I found that one of the resistors in the tape-in circuit was getting very warm - when I swapped that out for a resistor of the same value but higher current rating, things got a lot more reliable. I still seem to have to use a higher volume level than on a normal Spectrum - I suspect that's due to the Spectrum's ULA working at 5v, and the Max3232 RS232 UART that I'm using to buffer input between tape and FPGA is expecting a higher voltage.

Finally, I've started hooking up the MMC slots so I can start implementing DivMMC and getting ESXDOS working. I'll have to do some more research on when exactly ESXDOS needs to be paged in and out of the ROM area (it reacts to an NMI switch, pages in as a shadow ROM on RST8, and has tape traps at least... and I have no idea of what triggers them - or what triggers a page out). I also need to know how the DivMMC does its ROM/RAM paging. If anyone can help, leave me a comment or seek me out on WoSF.

I've already done a bunch of Googling and searching threads on the World of Spectrum forums... Looks like I'll have to ask the designers/coders (or an emulator author perhaps) directly. Andrew Owen's already managed to give me a certain amount of insight, but I'm far from clear on all points!

Finally, a picture of ZX Prism as it currently looks. Please ignore any dodgy soldering you might notice. I've just got a new soldering iron, and it's a huge upgrade from my last one!

The dev board is a Black Gold Cyclone IV kit, which is very nice but you have to translate all the documentation from Chinese! This dev board comes with 256K of 16 bit SRAM, a similar sized FLASH, about 4MB of SDRAM, RS232, VGA, USB, PS/2 (which was broken on my board), and a LAN socket as well as the 7 segment displays and LCD screen. The veroboard contains the analogue circuitry for tape-in and (untested as yet) tape-out, the dual MMC sockets, and also the break-out connector for the PS/2 and VGA daughterboard (not pictured) which replace the implementation of each on the main dev board (the PS/2 socket was broken on delivery, and the on-board VGA was 8 colour only)

Sunday, August 3, 2014

ULAplus support

Spent the weekend tying up some loose ends and working on the user-definable palettes. ULAplus support is pretty much working now (it's currenlty missing the ability to read the palette back, and the ULAplus specification includes the Timex screen modes which aren't implemented yet).

I've also hooked the SRAM up, so Prism now has 48K. There's 512K SRAM on the dev board and Prism will have access to all that once I've implemented bank switching.