Has anyone examined how large the stack is in Infocom’s various interpreters for 8-bit computers?
Has anyone examined how much stack space the larger Infocom games require? (AMFV, Bureaucracy, Zork Zero…)
Since we now have access to the source code for one of Infocom’s interpreters [1] I guess we can check:
PAGE
SBTTL "--- MEMORY ORGANIZATION ---"
TRUE EQU $FF
FALSE EQU 0
DSTART EQU 0 ; START OF DIRECT-PAGE RAM
MSTACK EQU MSTART+$FE ; TOP OF MACHINE STACK (254 BYTES)
IOBUFF EQU MSTART+$100 ; 256-BYTE DISK I/O BUFFER
ZSTACK EQU MSTART+$200 ; Z-STACK (255 WORDS)
ZSTAKL EQU 255 ; LENGTH OF Z-STACK IN WORDS
TOPSTA EQU (2*ZSTAKL)+ZSTACK ; TOP OF Z-STACK
PTABLE EQU MSTART+$400 ; PAGING TABLE ($140 BYTES/$A0 WORDS)
LRUMAP EQU MSTART+$550 ; TIMESTAMP MAP ($A0 BYTES)
LOCALS EQU MSTART+$600 ; LOCAL VARIABLE STORAGE (32 BYTES)
BUFFER EQU MSTART+$620 ; I/O LINE BUFFER (32 BYTES)
BUFSAV EQU MSTART+$640 ; I/O AUX BUFFER (32 BYTES)
ZIP EQU MSTART+$700 ; START OF EXECUTABLE CODE
ZCODE EQU ZIP+$1700 ; START OF Z-CODE (ASSUME 5.75K ZIP)
So at least for V3, apparently 255 words was deemed enough for the stack?
I don’t know about the larger games, though. You may be able to test A Mind Forever Voyaging, because of a bug in Library Mode. Every time you move the cursor it leaks a few bytes - I’m not sure exactly how much - of stack space that isn’t released until you exit Library Mode. (Apparently it’s because the main Library Mode loop calls functions without handling their return values, causing them to remain on the stack.)
So if you start the game, immediately go to Library Mode and hold down the “N”, eventually the game will crash. The game starts at 7:07 pm. Each time you move the cursor the clock advances by one minute. (Which doesn’t really make sense from a story point of view, but…) I only have access to the DOS and Mac versions in emulation, though, so I don’t have much to compare with, but:
The DOS interpreter crashes around 8:54 pm, i.e. less than 120 moves.
The Mac interpreter crashes around 8:52 pm, i.e. less than 120 moves.
Frotz 2.50 crashes around 11:00 pm, i.e. less than than 240 moves.
Frotz defines zword stack[STACK_SIZE], where STACK_SIZE is 1024. So perhaps a reasonable guess would be that Infocom’s had at most 512 words, and quite possibly less? (After all, it might not crash immediately when the stack space runs out.)
I have no idea about the V6 games.
[1] https://retrotinker.blogspot.com/2018/02/z-intepreter-source-for-coco-recovered.html
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