Friday, January 8, 2021

Dragon's Lair's software watchdog

I've been rewriting Daphne's Z80 emulator (from scratch) and testing it on Dragon's Lair. I've got it far enough along so that it loops at the disc routine at 0x231 waiting for an LD-V1000 strobe. While I had it looping, I was checking out some performance metrics to see how fast my new Z80 emu was running. To my shock, after about 1 minute, program execution moved away from that endless loop. After a few hours of tracking down why, I discovered this software watchdog that I never knew existed.

(my disassembly below)

ROM:19E3 DecrementWatchdog: ; CODE XREF: IRQ+1BC↑j
ROM:19E3 ld hl, A0DE_WatchDogCounter
ROM:19E6 call DecrementMem16 ; Decrements a 16-bit memory location
ROM:19E6 ; Flags are set according to SBC rules.
ROM:19E6 ; HL: pointer to 16-bit memory value
ROM:19E6 ; DE and BC are clobbered
ROM:19E9 jr nc, CheckSoundThenExitIrq
ROM:19EB jp DoSoftReset ; watchdog has not been fed. we're stuck in an endless loop.

ROM:1FCF DoSoftReset: ; CODE XREF: IRQ+1CB↑j
ROM:1FCF ld sp, 0A3FFh ; blow away all stack info; start over.
ROM:1FD2 ld hl, 0A030h
ROM:1FD5 ld de, 0A031h
ROM:1FD8 ld bc, 3CFh
ROM:1FDB ld (hl), 0
ROM:1FDD ldir ; this appears to clear mosts of the game data except the boot-up stuff
ROM:1FDF call ResetWatchdogCounter ; resets watchdog counter back to max value (about 70 seconds)
ROM:1FDF ; (this must be called regularly to avoid a soft system reset)
ROM:1FE2 ei
ROM:1FE3 jp loc_1244 ; this is an alternative to "RETI"; essentially giving up on recovering

ROM:1C2D ; =============== S U B R O U T I N E =======================================
ROM:1C2D
ROM:1C2D ; resets watchdog counter back to max value (about 70 seconds)
ROM:1C2D ; (this must be called regularly to avoid a soft system reset)
ROM:1C2D
ROM:1C2D ResetWatchdogCounter: ; CODE XREF: START+10↑p
ROM:1C2D ; sub_15B4+F5↑p ...
ROM:1C2D ld hl, 8AAh
ROM:1C30 ld (A0DE_WatchDogCounter), hl
ROM:1C33 ret
ROM:1C33 ; End of function ResetWatchdogCounter
ROM:1C33
ROM:1C34
ROM:1C34 ; =============== S U B R O U T I N E =======================================

Tuesday, December 29, 2020

Dragon's Lair (and Space Ace) boot-up explained

I occasionally get questions about whether Dexter is 'the problem' when a Dragon's Lair or Space Ace doesn't work. Here's a video to shed some light on where boot problems may be.

Tuesday, December 8, 2020

ASUS G75VW laptop - replacing small internal fan connector

When I was taking my Asus G75VW laptop apart (an older laptop but still quite useful!), I noticed one of the fan power connectors was falling apart. It broke as I removed it.

After much searching, I found an "exact" replacement:

TE Connectivity part 440146-4

or Digikey part A98724-ND

Hopefully this helps someone else in the future, as this was quite difficult for me to locate.

Wednesday, November 18, 2020

Dockerized ld-decode (rev6)

Easy way to run ld-decode (rev6) using docker:

docker pull mpownby/ld-decode:rev6

Launching with Ubuntu (with X windows shared, allows 'ld-analyse' to be run)

docker run --rm -v $HOME:/shared -v /tmp/.X11-unix:/tmp/.X11-unix -e DISPLAY -e "QT_X11_NO_MITSHM=1" -ti mpownby/ld-decode:rev6 /bin/bash

Launching with Windows (or other host where you only want to use the command-line tools)

docker run --rm -v /c/your/host/dir/here:/shared -e DISPLAY -e "QT_X11_NO_MITSHM=1" -ti mpownby/ld-decode:rev6 /bin/bash

In the above example, c:\your\host\dir\here is written as /c/your/host/dir/here . Modify this to point to a folder that you want the docker container to have access to (ie the place where your domesday capture lives). From within the container, this can be accessed by going to '/shared'.

Caveat: I couldn't get the volume mapping to work with an external USB hard drive. But it worked with my internal SATA hard drives.

Caveat: Beware that Docker advises that this may cause performance problems but it seems to be working fine for me in my tests.

Once the container starts, run these commands:

cd ld-decode
(optional) cat /usage.txt
python3 ld-decode.py /shared/your_domesday_file_here.ldf /shared/output

This will begin decoding the .ldf file (compressed domesday capture) from the beginning.

NOTE: 'output' is the prefix of the output files that will be created. You can use a different name but my instructions will assume you are using 'output'.

IMPORTANT: Most domesday captures were recorded while the laserdisc player is spinning up so the beginning of the capture will be "throw away". You need to let the decode run until you see the CAV frames go back to frame 1. Here is an example of how this looks:

(stuff before this removed for brevity)

file frame 626 CAV frame 244

file frame 627 CAV frame 245

file frame 628 CAV frame 246

file frame 629 CAV frame 247

file frame 630 CAV frame 248

file frame 631 CAV frame 249

file frame 632 CAV frame 250

file frame 633 CAV frame 251

file frame 634 CAV frame 252