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Applications and language tools

OaSis includes a kernel-mode text editor, a built-in x86 assembler, and the occ subset C compiler. These tools run inside the VGA shell and write output through kernel services.

Text editor

Source: src/kernel/apps/editor.c

The editor stores up to 4 KiB of text and supports navigation, editing, saving, and scrolling.

Key	Action
`Ctrl+X`	Exit after saving changes.
`Ctrl+S`	Save through the VFS.
Arrow keys	Move the cursor.
`Home` and `End`	Move to the start or end of a line.
`Backspace` and `Delete`	Remove characters.
`Enter`	Insert a newline.
`Tab`	Insert four spaces.

The editor renders multiple lines from the scroll position. A status bar shows the filename, line, and column.

Built-in assembler

Source: src/kernel/apps/asm.c

The assembler accepts a subset of 32-bit x86 instructions, labels, data directives, external symbols, and the times directive. It emits machine code into the CODE_VIRT execution area.

Assembly flow

asm_assemble(code, &exec_addr)
1. Copy the source into the input buffer.
2. Process each line.
   - A line ending in ':' defines a label.
   - An instruction is parsed and encoded.
3. Apply forward-reference patches.
4. Allocate pages at CODE_VIRT (0x40000000).
5. Copy the generated bytes into CODE_VIRT.
6. Return the execution address.

Labels and patches

Labels store names and byte offsets. Patches store the byte position, instruction origin, target label, and relocation type.

static struct {
    char name[32];
    int position;
} labels[MAX_LABELS];

static struct {
    int position;
    int origin;
    char target[32];
    int type;
} patches[MAX_PATCHES];

Supported instruction groups

mov, add, sub, cmp, xor, and, or
jmp, je, jne, jg, jl, jge, jle
call, push, pop, inc, dec
imul, idiv, cdq, div, neg, test, movzx
Conditional setcc and cmovcc instructions
int, nop, hlt, sti, cli, pusha, and popa
db and times

Occ compiler

The occ compiler is a small subset C compiler built from src/kernel/lib/lexer.c, parser.c, and codegen.c.

It supports integers, characters, functions, parameters, arrays, expressions, if, while, for, printf, and allocation helpers. It emits x86 assembly that the built-in assembler can execute.

Compiler pipeline

C source
  -> lexer: source characters become tokens
  -> parser: tokens become an AST
  -> code generator: AST becomes x86 assembly
  -> assembler: assembly becomes executable bytes

The compiler is intentionally limited. It is a teaching tool, not a complete C implementation. See the shell guide for occ, lex, parse, nasm, and user commands.