Spice86

Spice86 is an experimental emulator, whose support for Stunts is being worked on as of 2025. At present (2025-03-01) it can start the game, but no sound is played (start with -a /ns to avoid wasting cycles on that) Additionally, one must give the parameter --InstructionsPerSecond 5000000.

The reason for this workaround is that Stunts activates interrupts in the middle of the int 8 handler (see here), so another int 8 can could be triggered in between. This interrupt corresponds to the programmable interrupt timer, which Stunts sets at 100 Hz. So if the int 8 handler needs more than 1/100 seconds to run, disaster ensues. This probably never happens on real CPUs, but might occur if the emulator's simulated CPU is too slow with respect to the simulated PIT.

Here a dump the relevant snippets, which should eventually go elsewhere

seg012.asm-3514-    mov     dx, 2E9Ch      ; Frequency for the PIT. 1193182 / 0x2e9c ~= 99.998 Hz
[...]
seg012.asm-3555-    mov     al, 0B6h        ; Set up channel 2 (PC Speaker). Unrelated, included for reference.
seg012.asm:3556:    out     43h, al         ; Set channel 2 to opmode = 3 (square wave)
[...]
seg012.asm-3582-    mov     al, dl
seg012.asm:3583:    out     40h, al         ; Timer 8253-5 (AT: 8254.2).
seg012.asm-3584-    mov     al, dh
seg012.asm:3585:    out     40h, al         ; Timer 8253-5 (AT: 8254.2).
--
seg012.asm-3621-    out     21h, al         ; Interrupt controller, 8259A.
seg012.asm-3622-    sti
seg012.asm-3623-    mov     al, 0           ; Reset frequency to 1193182 / 65536 ~= 18.2 Hz
seg012.asm:3624:    out     40h, al         ; Timer 8253-5 (AT: 8254.2).
seg012.asm:3625:    out     40h, al         ; Timer 8253-5 (AT: 8254.2).

Relevant docs

• https://wiki.osdev.org/Programmable_Interval_Timer
• https://en.wikibooks.org/wiki/X86_Assembly/Programmable_Interval_Timer