Input and interrupts

The game combines keyboard polling, mouse callbacks and a hardware timer. key_cmd() and menu routines wait for commands; CONTROL.C interprets commands during unit movement. Keyboard helpers include entries loaded from MISC.EXE, whose source is MISC.ASM.

The numeric keypad supplies eight movement directions. Letter commands include B to found a city, F to fortify, S for sentry, R for roads, I for irrigation, M for mining and G for a destination. Context determines which commands are available. The tests and resulting orders are in ai_turn().

Mouse

MOUSE.INC talks to the installed DOS mouse driver through INT 33h. The game uses its own sprite cursor through graphics-driver calls. mouse_on() and mouse_off() in GAME.C keep a nesting count, so paired calls preserve the visibility state across nested drawing operations. Cursor drawing saves the pixels underneath and restores them on erasure. The MCGA driver limits that saved area to 512 bytes.

Timer

TIMER.INC, included by RESIDENT.ASM, installs an INT 08h handler and programs the hardware timer. It uses a private interrupt stack, calibrates against vertical retrace, and chains to the saved handler as accumulated time requires. The sound driver can request a faster rate for pitch sweeps. Normal timer processing services sound and palette animation.

restore_timer() restores the saved vector and timer programming. This is a direct IRQ0 hook, rather than an INT 1Ch callback running only at the standard DOS timer rate.

Break handling

install_break_handlers() in GAME.C saves the INT 1Bh and INT 23h vectors and replaces them with an empty handler. restore_break_handlers() restores them at exit. Disk operations have separate error paths in FILEIO.C and SAVE.C; they should be read separately from break handling.