# System Call Reference

Complete reference for ModuOS system calls.

## Calling Convention

**Registers**:
```
RAX: Syscall number
RDI: Argument 1
RSI: Argument 2
RDX: Argument 3
RCX: Argument 4
R8:  Argument 5
R9:  Argument 6

Return: RAX (0 or positive on success, negative on error)
```

**Trigger**: `int 0x80`

## Process Control

### SYS_EXIT (0)
```c
void exit(int status);
```
Terminate current process with exit code.

### SYS_GETPID (5)
```c
int getpid(void);
```
Get current process ID.

**Returns**: Process ID

### SYS_FORK (6)
```c
int fork(void);
```
Create child process (not fully implemented).

**Returns**: 0 in child, child PID in parent, negative on error

### SYS_EXEC (7)
```c
int exec(const char *path, char **argv);
```
Execute program from file.

**Returns**: Negative on error (doesn't return on success)

### SYS_YIELD (11)
```c
void yield(void);
```
Yield CPU to other processes.

### SYS_SLEEP (10)
```c
int sleep(uint64_t milliseconds);
```
Sleep for specified time.

**Returns**: 0 on success

## File I/O

### SYS_OPEN (3)
```c
int open(const char *path, int flags);
```
Open file.

**Flags**:
- `O_RDONLY` (0): Read only
- `O_WRONLY` (1): Write only
- `O_RDWR` (2): Read/Write

**Returns**: File descriptor, or negative on error

### SYS_CLOSE (4)
```c
int close(int fd);
```
Close file descriptor.

**Returns**: 0 on success, negative on error

### SYS_READ (1)
```c
int read(int fd, void *buffer, size_t count);
```
Read from file.

**Returns**: Bytes read, or negative on error

### SYS_WRITE (2)
```c
int write(int fd, const void *buffer, size_t count);
```
Write to file.

**Returns**: Bytes written, or negative on error

## Memory Management

### SYS_BRK (14)
```c
void* brk(void *addr);
```
Change data segment size.

**Returns**: New break address, or NULL on error

### SYS_MMAP (12)
```c
void* mmap(void *addr, size_t length, int prot, int flags);
```
Map memory region.

**Returns**: Mapped address, or NULL on error

### SYS_MUNMAP (13)
```c
int munmap(void *addr, size_t length);
```
Unmap memory region.

**Returns**: 0 on success, negative on error

## Error Codes

```c
#define ESUCCESS    0   // Success
#define EPERM      -1   // Operation not permitted
#define ENOENT     -2   // No such file or directory
#define EBADF      -3   // Bad file descriptor
#define ENOMEM     -4   // Out of memory
#define EINVAL     -5   // Invalid argument
#define ENOSYS     -6   // Function not implemented
```

## Example Usage

```c
// Open and read file
int fd = open("/file.txt", O_RDONLY);
if (fd < 0) {
    // Error handling
    return;
}

char buffer[1024];
int bytes = read(fd, buffer, sizeof(buffer));
close(fd);

// Exit program
exit(0);
```

## Next Steps

- [System Calls](System-Calls.md) - Implementation details
- [Kernel API](Kernel-API.md) - Kernel functions