System Calls

System Calls

System calls in MVM (Micro Virtual Machine) allow your assembly programs to request services from the underlying operating system or environment. These services include file operations, gathering system information, and more.

Making System Calls

The SYSCALL instruction executes a system call. Before executing SYSCALL, you must set the following registers:

• S0: The system call ID (a number that identifies the specific system call).
• S1 - S3: Arguments for the system call (if any). The meaning of these arguments depends on the specific system call being invoked.

System calls typically return a result in register R2. Some system calls may not return a value. All errors encountered during system calls will immediately terminate the program.*

LIT S0 14      // System call ID for "time" (get current time)
SYSCALL        // Execute the system call. The current time is now in R2.
MOV R2 G1    // Store the returned time from R2 into general register G1.

System Call Table

The System Call Table provides a comprehensive list of all available system calls, their IDs, arguments, and return values.

Common System Calls

This section describes some of the frequently used system calls:

• File I/O:

  • openFile(path) (ID: 3): Opens the file specified by the null-terminated string at the memory address in S1. Returns a file descriptor in R2.
  • readFile(fd) (ID: 1): Reads data from the file identified by the file descriptor (fd) in S1. Stores the starting address of the read data in memory in R2.
  • writeFile(fd, buffer) (ID: 2): Writes data to a file. holds the file descriptor, and S2 contains the starting address of the buffer in memory that holds the data to be written.
  • closeFile(fd) (ID: 4): Closes the file specified by the file descriptor in S1.

• Process Management:

  • exit(exitCode) (ID: 5): Terminates the MVM program with the exit code provided in S1.

• System Information:

  • time() (ID: 14): Returns the current time in milliseconds in R2.
  • getpid() (ID: 16): Returns the process ID of the MVM instance in R2.
  • getuid() (ID: 17): Returns the user ID in R2.

• Input/Output:

  • writeIo(stringAddr) (ID: 24): Prints the null-terminated string at the memory address in S1 to the console.
  • readIo() (ID: 25): Reads a line from standard input and stores it as a null-terminated string in memory. Returns the starting address of the string in R2.

• Array Operations:

  • createArray(size) (ID: 26): Creates an array of the specified size (in S1). Returns the base address of the allocated array in R2.
  • arraySet(arrayAddr, index, value) (ID: 27): Sets the element at index of the array at arrayAddr to value. Arguments: arrayAddr in S1, index in S2, value in S3.
  • arrayGet(arrayAddr, index) (ID: 28): Returns the value of the element at index in the array at address arrayAddr. Arguments are passed in the same order as arraySet. The element's value is returned in R2.

Example: Writing to a File

// ...get file descriptor (fd) in R1 using openFile...

// String to write (must be null-terminated)
STR S2 "Hello, file!\n"  // S2 holds the string's address

LIT S0 2    // System call ID for writeFile
MOV R2 S1   // File descriptor (fd) as the first argument in S1
CPY S2 S2    // Address of the string to write as the second argument in S2
SYSCALL      // Call writeFile