10  i_system: Core System Interface

This tour examines i_system.h and its implementations — the core system interface providing platform-specific services to the Doom engine.
It covers system initialization, memory management, timing, input, and error handling.

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Next we’ll look at I_Init for system startup and I_ZoneBase for memory allocation setup.

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10.0.1 📄 File: i_system.h (lines 34–36)

I_Init prototype for system-wide initialization.

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10.0.2 📄 File: i_system.c (lines 104–111)

I_Init implementation — prepares platform systems. Note: graphics are initialized separately.

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10.0.3 📄 File: i_system.h (lines 40–41)

I_ZoneBase provides a memory pointer and size for zone allocator use.

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10.0.4 📄 File: i_system.c (lines 76–80)

I_ZoneBase allocates the engine’s heap, defaulting to 6MB.

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Next we’ll look at DOOM’s timing and input functions, which handle game synchronization and event processing.

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10.0.5 📄 File: i_system.h (lines 44–45)

I_GetTime returns game time in tics.

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10.0.6 📄 File: i_system.c (lines 88–100)

I_GetTime converts system time to tics using TICRATE.

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10.0.7 📄 File: i_system.h (lines 56–64)

I_StartFrame and I_StartTic handle frame and tic timing; trigger input processing.

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10.0.8 📄 File: i_video.c (lines 181–187)

I_StartFrame is a no-op in the X11 port.

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10.0.9 📄 File: i_video.c (lines 307–338)

I_StartTic processes X11 events, and recenters the mouse if needed.

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10.0.10 📄 File: i_system.h (lines 70–74)

I_BaseTiccmd prepares an empty input command structure (ticcmd_t).

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10.0.11 📄 File: d_ticcmd.h (lines 36–44)

ticcmd_t contains player input: movement, angle, chat, buttons, network sync.

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10.0.12 📄 File: i_system.c (lines 64–68)

I_BaseTiccmd returns a zero-initialized command structure.

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10.0.13 📄 File: i_system.h (lines 77–79)

I_Quit prototype for game shutdown sequence.

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10.0.14 📄 File: i_system.c (lines 116–124)

I_Quit implementation: network cleanup, sound/music stop, graphics shutdown.

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10.0.15 📄 File: i_system.h (lines 82–87)

I_AllocLow and I_Tactile — memory allocation and haptics interface.

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10.0.16 📄 File: i_system.c (lines 147–154)

I_AllocLow wraps malloc + memset on Unix.

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10.0.17 📄 File: i_system.c (lines 54–62)

I_Tactile is a stub under Linux — all params unused.

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10.0.18 📄 File: i_system.h (lines 88–90)

I_Error prototype — variadic error reporter.

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10.0.19 📄 File: i_system.c (lines 162–172)

I_Error shows a fatal message, performs cleanup, and exits.

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10.0.20 📄 File: i_system.c (lines 167–171)

Formats error messages using vfprintf with variadic args.

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10.0.21 📄 File: i_system.c (lines 173–177)

Flushes stderr, ends demo recording via G_CheckDemoStatus.

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10.0.22 📄 File: i_system.c (lines 179–182)

Final shutdown steps: network and graphics termination.

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10.0.23 📄 File: d_main.c (lines 611–614)

At startup, if $HOME isn’t set, I_Error prompts and exits.

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10.0.24 📄 File: d_net.c (lines 131–134)

I_Error used to report invalid tick state in ExpandTics.

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10.0.25 📄 File: r_data.c (lines 365–368)

Renderer uses I_Error to abort if a texture exceeds 64KB — prevents memory corruption.

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The system interface cleanly abstracts platform-specific details from Doom’s engine logic, supporting portability across OSes.