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---
title: PS-X OS
type: component
summary: Kernel of the PlayStation that provides its services as C functions.
category: System software
---

## Overview

The PS-X OS was developed for the R3000. The R3000 is the CPU of the PlayStation.[^os13]

The kernel is a set of services (subroutines) that control the R3000 and the PlayStation hardware. Each service is a C language function.[^os13]

The PS-X OS takes at most 64 kilobytes of RAM.[^os13]

The OS discloses its system tables. Applications can extend the OS and read its internal data.[^os13]

The OS leaves out the checks for prohibited operations that other operating systems perform. The application must perform these checks.[^os13]

The OS has no user interface for operating the hardware directly. The debug monitor in the debugging environment is the exception.[^os23]

## Tasks

The PS-X OS is a multi-tasking operating system. It starts in single task mode.[^os43]

A program that does not start more threads stays in single task mode.[^os43]

## Boot

The OS has two start modes. The DIP switches of the target box select the mode at power on or reset.[^os23]

In hardware mode the OS boots as the PlayStation hardware does.[^os23]

The application boot jumps to a fixed address in ROM. The boot code then checks the CD-ROM drive and the other hardware connected to the PlayStation.[^os23]

The application boot then checks the type of disc.[^os23]

On the right disc the boot code reads the file [SYSTEM.CNF](/system/system-cnf/). The boot code then starts the program named in SYSTEM.CNF.[^os23]

With no disc the application boot runs a looping demonstration.[^os23]

On a development system booted from a PC/AT host, the kernel takes its default configuration and the remote debugger starts. The executable is then loaded by hand.[^os23]

## File System

The file system assumes that files are on CD-ROM. It is based on ISO 9660 level 1.[^os24]

File systems are accessed through device drivers. Several file systems can therefore coexist.[^os14]

A file name is an 8-character base name and a 3-character extension. A period separates the two. This is the MS-DOS name format.[^os24]

Names use 1-byte alphanumeric characters. Katakana and kanji cannot be used.[^os24]

Subdirectories cannot be created. All files are in the root directory, and the number of files there has no limit.[^os24]

## Memory Map

The memory map of the PS-X OS is the [memory map](/hardware/memory-map/) of the CPU.[^os25]

Interface registers of peripheral devices are mapped in logical space as memory mapped I/O.[^os25]

Memory can be accessed in units of 1 to 4 bytes.[^os25]

An interface register must be accessed in its own unit size. Any other size raises a bus error.[^os25]

## System Tables (ToT)

The OS discloses its internal system tables through the ToT (Table of Tables) structure. ToT is at address 0x00000100.[^os26]

ToT has the following entries:

Table: System tables

| Entries | Nature |
| --- | --- |
| 0 | Queue header requires interrupt processing[^os26] |
| 1 | Task state queue header |
| 2 | Task management block |
| 3 | System reserved |
| 4 | Event management block |
| 5 to 31 | System reserved |

The structure is defined in include/kernel.h. Each entry holds the lead address and the size in bytes of one system table.[^os26]

```
struct ToT {                  /*system table table*/
    unsigned long *head;      /*system table lead address*/
    long size;                /*system table size (byte)*/
};
```

## Function Calls

A C function receives its first 4 arguments in A0 to A3.[^os28]

Space for those 4 arguments is also reserved on the stack. Further arguments are passed on the stack.[^os28]

A return value of less than 32 bits is in V0. For a 64-bit double the high 32 bits are in V1.[^os28]

An external interrupt or an error other than a debug error saves the program counter to the EPC register of coprocessor 0.[^os28]

The CPU then jumps to 0x00000080.[^os28]

The [CPU](/hardware/cpu/) page lists the register assignments.

## Services

The OS services are called application programmer interfaces (API). A program that calls the OS links their function libraries.[^os33]

The API has the following services:

Table: PS-X OS services

| Service | Content |
| --- | --- |
| Module Management Service | Loads and executes user application programs[^os33] |
| Event Management Service | Controls program execution triggered by asynchronous events |
| Controller Service | Supports up to 64 controllers of up to 16 buttons each |
| I/O Management Service | Low-level input and output to files and logical devices |
| Thread Management Service | Threads of register states, stack areas and CPU states |
| Other Services | I-cache and R3000 setup, enabling and disabling interrupts |
| Standard C Library | K&R standard C functions such as string operations and standard I/O |
| Root Counter Management Service | Counters for timing[^os34] |

All file control on the PlayStation goes through the I/O management service.[^os33]

## Root Counters

The PS-X OS has 4 root counters. Each counts automatically.

Table: Root counters

| Counter | Counts |
| --- | --- |
| 1 | Display pixels[^os34] |
| 2 | Horizontal synchronization |
| 3 | System clock |
| 4 | Vertical synchronization |

A counter counts up from zero. At its target value it raises an interrupt, clears to zero and counts again.[^os34]

Every counter except vertical synchronization takes a 16-bit target value. The target value of vertical synchronization is fixed at 1.[^os34]

The root counter and the event management service together detect the start of the vertical blanking period.[^os44]

## Programming Notes

EnterCriticalSection() stops interrupts.[^os43]

An overlay runs as a child process of a resident module. The application must load and start the overlay itself.[^os43]

The PlayStation has no character generator and no fonts. An application draws its own text as graphics.[^os43]

The C library has heap functions. The OS does not initialize the heap.[^os44]

The program passes the start address and size of the heap to the heap initialization function.[^os44]

The entry point of an executable is set by a function name at link time. The default is main(), and the entry function takes no argument.[^os44]

A program needs a stack pointer value as well as a start address. A stack pointer at an address that cannot be accessed causes a bus error.[^os45]

Standard output is file descriptor 0 and standard input is file descriptor 1. Both use RS232C channel 0. Its device name is tty00.[^os45]

The tty00 port uses 8-bit characters, 1 stop bit, no parity and hardware flow control.[^os45]

## References

[^os13]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 1-3.
[^os14]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 1-4.
[^os23]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 2-3.
[^os24]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 2-4.
[^os25]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 2-5, Table 2-2.
[^os26]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 2-6, Table 2-3.
[^os28]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 2-8.
[^os33]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 3-3.
[^os34]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 3-4.
[^os43]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 4-3.
[^os44]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 4-4.
[^os45]: Sony Computer Entertainment Inc. PlayStation Operating System. August 1998. Page 4-5.