PS-X OS

Kernel of the PlayStation that provides its services as C functions.

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Overview

The PS-X OS was developed for the R3000. The R3000 is the CPU of the PlayStation.¹

The kernel is a set of services (subroutines) that control the R3000 and the PlayStation hardware. Each service is a C language function.¹

The PS-X OS takes at most 64 kilobytes of RAM.¹

The OS discloses its system tables. Applications can extend the OS and read its internal data.¹

The OS leaves out the checks for prohibited operations that other operating systems perform. The application must perform these checks.¹

The OS has no user interface for operating the hardware directly. The debug monitor in the debugging environment is the exception.²

Tasks

The PS-X OS is a multi-tasking operating system. It starts in single task mode.³

A program that does not start more threads stays in single task mode.³

Boot

The OS has two start modes. The DIP switches of the target box select the mode at power on or reset.²

In hardware mode the OS boots as the PlayStation hardware does.²

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.²

The application boot then checks the type of disc.²

On the right disc the boot code reads the file SYSTEM.CNF. The boot code then starts the program named in SYSTEM.CNF.²

With no disc the application boot runs a looping demonstration.²

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.²

File System

The file system assumes that files are on CD-ROM. It is based on ISO 9660 level 1.⁴

File systems are accessed through device drivers. Several file systems can therefore coexist.⁵

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.⁴

Names use 1-byte alphanumeric characters. Katakana and kanji cannot be used.⁴

Subdirectories cannot be created. All files are in the root directory, and the number of files there has no limit.⁴

Memory Map

The memory map of the PS-X OS is the memory map of the CPU.⁶

Interface registers of peripheral devices are mapped in logical space as memory mapped I/O.⁶

Memory can be accessed in units of 1 to 4 bytes.⁶

An interface register must be accessed in its own unit size. Any other size raises a bus error.⁶

System Tables (ToT)

The OS discloses its internal system tables through the ToT (Table of Tables) structure. ToT is at address 0x00000100.⁷

ToT has the following entries:

The structure is defined in include/kernel.h. Each entry holds the lead address and the size in bytes of one system table.⁷

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 A3.⁸

Space for those 4 arguments is also reserved on the stack. Further arguments are passed on the stack.⁸

A return value of less than 32 bits is in V0. For a 64-bit double the high 32 bits are in V1.⁸

An external interrupt or an error other than a debug error saves the program counter to the EPC register of coprocessor 0.⁸

The CPU then jumps to 0x00000080.⁸

The 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.⁹

The API has the following services:

All file control on the PlayStation goes through the I/O management service.⁹

Root Counters

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

A counter counts up from zero. At its target value it raises an interrupt, clears to zero and counts again.¹⁰

Every counter except vertical synchronization takes a 16-bit target value. The target value of vertical synchronization is fixed at 1.¹⁰

The root counter and the event management service together detect the start of the vertical blanking period.¹¹

Programming Notes

EnterCriticalSection() stops interrupts.³

An overlay runs as a child process of a resident module. The application must load and start the overlay itself.³

The PlayStation has no character generator and no fonts. An application draws its own text as graphics.³

The C library has heap functions. The OS does not initialize the heap.¹¹

The program passes the start address and size of the heap to the heap initialization function.¹¹

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.¹¹

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.¹²

Standard output is file descriptor 0 and standard input is file descriptor 1. Both use RS232C channel 0. Its device name is tty00.¹²

The tty00 port uses 8-bit characters, 1 stop bit, no parity and hardware flow control.¹²

References