simso — Operating System Simulator

A small terminal simulation of a mono‑tasking operating system: a round‑robin CPU scheduler with Banker's‑style deadlock avoidance, a fixed‑size memory allocator, and separate I/O and resource‑wait queues. It runs entirely in the terminal as a live ncurses dashboard.

The program began life in the 1990s as a university project written for MS‑DOS / Turbo‑C, using the BGI graphics library (libregra.h) and a mouse library (raton.h) for its interface. This version is a port of that original to a modern Linux terminal: the scheduling logic is preserved, while the graphics/mouse/DOS layer has been replaced with ncurses and keyboard input.

What it does

Every tick, the simulator schedules processes through their full lifecycle and draws the current system state:

• A memory bar of 64 cells, coloured per owning process.
• Available resources (three pools: A, B, C).
• Four queues: Waiting queue, Ready, I/O queue, and the resource‑wait queue (Resources).
• A PCB panel for the process currently on the CPU — state, priority, remaining execution time, maximum/assigned resources, and I/O bursts.

Processes request and release resources; the Banker's‑style check grants a request only if it keeps the system in a safe state, otherwise the process is parked on the resource‑wait queue until resources free up. Processes also periodically block for I/O and are re‑admitted from the waiting queue as memory and scheduler slots become available.

Requirements

• Linux on x86‑64 (or any POSIX system with ncurses).
• A C compiler — GCC/Clang, or Fil-C (filcc).
• ncurses development headers (libncurses-dev on Debian/Ubuntu). Not needed for the Fil-C build — see below.
• A terminal at least 80×24; the program refuses to start on anything smaller.

Build

With GCC (or Clang)

# install ncurses headers once (Debian/Ubuntu)
sudo apt-get install libncurses-dev

# standard build
gcc -O2 -Wall simso.c -o simso -lncurses

For a smaller release binary, drop debug info and strip symbols:

gcc -O2 -s simso.c -o simso -lncurses

Fancier flags (-O3, -flto, --gc-sections, -Os) don't help here: the program is tiny and spends nearly all its wall‑time asleep between animation frames, so -O2 is the sweet spot.

Note: GCC's -O2 flow analysis emits a few -Warray-bounds warnings on the early‑exit idiom in evitacion()/recursos(). They are false positives — the index stays in range at runtime, confirmed under AddressSanitizer/UBSan.

With Fil-C

Fil-C is a memory‑safe C/C++ implementation: it enforces bounds and use‑after‑free checks at runtime and aborts with a filc panic on any violation. It compiles this project unchanged:

filcc -O2 -g -o simso simso.c -lncurses

• Fil-C ships a pizlonated ncurses (6.5) in its pizfix, so no extra library setup is needed — the -lncurses above resolves against it automatically.
• Keep -g while developing: Fil-C's panic backtraces are far more useful with debug info (you get line numbers pointing at the offending access).

Troubleshooting: if the link fails with a wall of undefined reference to 'pizlonated_stdscr', 'pizlonated_initscr', etc., you forgot -lncurses — those are the ncurses symbols with nothing to resolve against. If -lncurses isn't found, try -lncursesw, or check what your pizfix provides (e.g. ls /opt/fil/*/lib | grep -i ncurses).

Run

./simso

On start you're prompted for:

1. Quantum — CPU time slice (1–50).
2. I/O time — how long an I/O burst blocks a process (1–50).
3. Finish mode:
   • 1 — stop after a chosen number N of processes complete.
   • 2 — run until all processes have finished.

Press q at any time during the simulation to abort cleanly. The terminal is always restored on exit.

At runtime ncurses needs $TERM set and a matching terminfo entry; the system default (/usr/share/terminfo) is fine.

Known quirk (preserved from the original): finish mode 2 exits almost immediately, after the first quantum. The original code uses the value 0 both to mean "run until all done" and as the finite‑mode countdown, so its numero == 0 exit test fires right away. This behavior was kept faithful to the source. To make mode 2 run to completion, capture the mode in a separate flag and gate that check on it.

Porting & hardening notes

The scheduling core is logically identical to the DOS original. Two kinds of change were made:

Interface (the point of the port). BGI graphics calls became an ncurses dashboard; the mouse + checkbox setup dialog became keyboard prompts; conio.h/dos.h calls (clrscr, gotoxy, delay, randomize) became their ncurses/stdlib equivalents. libregra.h and raton.h are gone entirely. All on‑screen text was also translated from Catalan/Spanish to English.

Memory‑safety fixes. DOS tolerated several out‑of‑bounds accesses that happened to overlap adjacent memory harmlessly; on Linux (and under Fil-C) those are crashes or undefined behavior. Each fix is commented inline in the source and is small and reversible:

• Writes past the 64‑cell memoria[] array clamped to valid bounds.
• Out‑of‑bounds PCB[12] reads in the resource‑search loops removed by bounding the search and ordering the && guard so it short‑circuits.
• The I/O‑burst array tiempoes[] was one slot too small for the index range the code uses; enlarged and fully initialised.
• The original rand() % rand() (which can divide by zero) replaced with an equivalent guarded helper.
• Free‑cell counters reset between passes to avoid a stale‑index read.

After these changes the program is clean across repeated AddressSanitizer + UBSan runs and both finish‑modes exit with status 0. Running it under Fil-C is a good ongoing check: the original would almost certainly have tripped Fil-C's guard on the first run.

A handful of unbounded sentinel searches (the != 'z' scans) were left as‑is; they rely on invariants that held throughout testing. If Fil-C ever panics in one of those, that's a genuine bug worth investigating rather than a false positive.

Project layout

Single translation unit:

• simso.c — everything (presentation layer + scheduler core), with a header comment on each function.

Credits

Original DOS/Turbo‑C project by Eduard G.Castello and Llorenç Llado. Linux/ncurses port and hardening applied to the original source.