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Mohiuddin Khan Inamdar edited this page Jun 21, 2026 · 3 revisions

Stage 3 — An Interactive Shell

The kernel learns to listen: a prompt, a line editor, a parser, and five commands — all without a libc.

Stage 2 ran a fixed demo. Stage 3 turns the kernel into something you can talk to: it prints a green shell> prompt, lets you edit a line, parses what you typed into a command and arguments, and dispatches to one of five built-in commands. The boot path is identical to Stage 2; what changes is entirely in the C kernel, which here is a shell.

  • Directory: os-c-with-shell/
  • Mode: 32-bit protected mode
  • Language: C + NASM
  • Banner on screen: SimpleShell OS v1.0

What's new vs Stage 2

Stage 2 Stage 3
Kernel role a UI demo an interactive command shell
Output model absolute putchar_at(x, y) a flowing cursor with newline handling and scrolling
Scrolling none scroll_screen() when output reaches row 25
Input character echo a line editor (get_line) with backspace and shift
Parsing none parse_command splits a command word from its arguments
String ops inline hand-written strcmp / strncmp
Commands none 5 built-ins: help, clear, echo, about, shutdown

shell.c is the kernel: kernel_main() does nothing but call shell_main(). The bootloader is the same as Stage 2 except it loads 15 sectors instead of 16 (boot.asm:34), sized to this stage's roughly 7.5 KB kernel.

The files

File Role
shell.c The entire kernel: VGA output with scrolling, polled keyboard, a line editor, a command parser, and five command handlers.
boot.asm The Stage 2 bootloader, loading 15 sectors.
kernel_entry.asm, linker.ld, keyboard.h, Makefile Unchanged from Stage 2 in structure.

The boot machinery (GDT, protected-mode switch, fixed load address) is exactly as described in Stage 2; this page focuses on the shell.

Code walkthrough: a flowing terminal

Stage 2 placed every character at an explicit (x, y). A shell needs text that flows — advancing the cursor, wrapping at the right edge, and scrolling when it runs off the bottom. putchar adds all of that on top of the Stage 2 primitive:

void putchar(char c, char color) {
    if (c == '\n') {
        cursor_x = 0;
        cursor_y++;
    } else if (c == '\b') {
        if (cursor_x > 0) {
            cursor_x--;
            putchar_at(' ', cursor_x, cursor_y, color);
        }
    } else {
        putchar_at(c, cursor_x, cursor_y, color);
        cursor_x++;
    }

    if (cursor_x >= 80) {       // wrap at the right edge
        cursor_x = 0;
        cursor_y++;
    }
    if (cursor_y >= 25) {       // ran off the bottom -> scroll
        scroll_screen();
        cursor_y = 24;
    }
}

scroll_screen copies every row up by one and clears the bottom row, working directly on the 0xB8000 framebuffer:

void scroll_screen() {
    volatile char* video = (volatile char*)VIDEO_MEMORY;
    for (int y = 0; y < 24; y++) {
        for (int x = 0; x < 80; x++) {
            int src = ((y + 1) * 80 + x) * 2;
            int dst = ( y      * 80 + x) * 2;
            video[dst]     = video[src];
            video[dst + 1] = video[src + 1];
        }
    }
    for (int x = 0; x < 80; x++) {        // clear last row
        int off = (24 * 80 + x) * 2;
        video[off]     = ' ';
        video[off + 1] = WHITE_ON_BLACK;
    }
}

print and println are thin loops over putchar. See vga-text-mode.md for the framebuffer layout.

Code walkthrough: the line editor

get_key is a trimmed version of Stage 2's reader — it polls the controller, handles the release bit, tracks only Shift, and returns either a translated character or 0 for events to ignore. get_line builds an editable line on top of it:

void get_line(char* buffer, int max_len) {
    int pos = 0;
    char key;
    while (pos < max_len - 1) {
        key = get_key();
        if (key == 0) continue;

        if (key == '\n') {                  // Enter: finish the line
            buffer[pos] = '\0';
            putchar('\n', WHITE_ON_BLACK);
            break;
        } else if (key == '\b') {           // Backspace: erase one char
            if (pos > 0) {
                pos--;
                putchar('\b', WHITE_ON_BLACK);
            }
        } else if (key >= 32 && key <= 126) {  // printable: echo + store
            buffer[pos++] = key;
            putchar(key, WHITE_ON_BLACK);
        }
    }
    buffer[pos] = '\0';
}

Only printable ASCII (32–126) is accepted into the buffer; Enter terminates, Backspace deletes, everything else is dropped. This is the smallest line editor that still feels like a real prompt. Scancode handling is detailed in scancodes.md and ps2-keyboard-8042.md.

💡 Tidbit: Backspace here both moves the buffer position back and paints a space at the new cursor location (via putchar's '\b' case), so the deleted character actually disappears from the screen rather than just moving the cursor. A surprising amount of "feels like a terminal" is small details like this.

Code walkthrough: parsing and dispatch

A typed line is split into a command word and an argument string by parse_command:

void parse_command(char* input, char* cmd, char* args) {
    int i = 0, j = 0;
    while (input[i] == ' ') i++;            // skip leading spaces
    while (input[i] && input[i] != ' ')     // first word -> cmd
        cmd[j++] = input[i++];
    cmd[j] = '\0';
    while (input[i] == ' ') i++;            // skip spaces
    j = 0;
    while (input[i])                        // the rest -> args
        args[j++] = input[i++];
    args[j] = '\0';
}

There is no libc, so even strcmp is hand-written:

int strcmp(const char* s1, const char* s2) {
    while (*s1 && (*s1 == *s2)) { s1++; s2++; }
    return *(unsigned char*)s1 - *(unsigned char*)s2;
}

The shell's main loop prints the prompt, reads a line, parses it, and runs an if/else if chain of strcmp comparisons:

void shell_main() {
    char input[MAX_CMD_LEN], cmd[64], args[MAX_CMD_LEN];
    clear_screen();
    println("SimpleShell OS v1.0", YELLOW_ON_BLACK);
    println("Type 'help' for available commands", CYAN_ON_BLACK);
    println("", WHITE_ON_BLACK);

    while (1) {
        print("shell> ", GREEN_ON_BLACK);
        get_line(input, MAX_CMD_LEN);
        parse_command(input, cmd, args);

        if (cmd[0] == '\0') {
            continue;
        } else if (strcmp(cmd, "help") == 0) {
            cmd_help();
        } else if (strcmp(cmd, "clear") == 0) {
            clear_screen();
        } else if (strcmp(cmd, "echo") == 0) {
            cmd_echo(args);
        } else if (strcmp(cmd, "about") == 0) {
            cmd_about();
        } else if (strcmp(cmd, "shutdown") == 0) {
            shutdown();
        } else {
            print("Unknown command: ", RED_ON_BLACK);
            println(cmd, RED_ON_BLACK);
        }
    }
}

The five commands are deliberately tiny:

Command Effect
help List the available commands.
clear Clear the screen and reset the cursor.
echo [text] Print the argument string in green.
about Show the OS name, version, and feature list.
shutdown Clear the screen, print "System halted.", then cli; hlt.

A full command reference for all stages is in command-reference.md.

⚠️ Caveat: Dispatch is a linear chain of strcmp calls, and cmd is copied into a fixed 64-byte buffer with no bounds check in parse_command. For a five-command shell driven by a 128-byte line this is fine, but it does not scale and does not defend against a pathologically long token. Stage 4 keeps the same dispatch style even as the command count quadruples.

How to build and run

From os-c-with-shell/:

make            # build the shell image
make run        # boot the shell in QEMU
make debug      # boot under QEMU with a GDB stub (-s -S)
make clean      # remove build artifacts

The build is the same shape as Stage 2 — freestanding gcc, nasm for the stubs, ld with the linker script, then cat boot.bin kernel.bin into the image — except shell.o takes the place of kernel.o. See building-and-running.md.

What it teaches

  • How a flowing terminal is built from an absolute-position putchar_at: cursor tracking, line wrap, and scrolling.
  • A minimal but usable line editor over a polled keyboard.
  • Tokenizing a command line and dispatching to handlers by string comparison.
  • Writing the string primitives (strcmp, strncmp) you normally take for granted, because there is no standard library underneath you.

Known limits

  • Five commands, linear dispatch. No command table yet; each command is an else if branch.
  • No history, completion, or aliases. Typing is single-line and immediate.
  • Only Shift among modifiers. Ctrl/Alt/Caps from Stage 2's reader are not tracked here.
  • No bounds check on the command token. The cmd buffer is fixed at 64 bytes and trusts the input length.

Next stage

Stage 4 asks "what does an OS actually do?" and answers with real subsystems: a CMOS real-time clock, a cooperative process model, a fixed-point calculator, and shell ergonomics (history, tab-completion, aliases) — growing from 5 commands to 20.

→ Stage 4 — Clock, Processes, and a Calculator

See also

MyOS-Simple Wiki

Stages

Concepts — boot

Concepts — protected mode

Concepts — hardware

Concepts — OS services

Reference

Guides

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