PS-X EXE file format: Header

Introduction

The header is always $800 bytes: the BIOS reads the first $800 bytes of the file, takes what it needs from the first $4C, and loads the rest. Every number is a 32-bit little-endian value. The layout follows the Nocash PSX-SPX page on PSX.EXE.

Fields

Offset	Size	Field	Meaning
`$000`	8	Signature	The ASCII text `PS-X EXE`, followed by eight zero bytes (`$008`–`$00F`).
`$010`	4	Initial PC	The address the program starts at. Usually `$80010000` or higher.
`$014`	4	Initial GP (R28)	The global pointer’s starting value; usually 0.
`$018`	4	Destination address	Where in RAM the program is loaded; usually `$80010000` or higher, because the first 64 KiB of RAM belong to the BIOS.
`$01C`	4	Size	The size of the program after the header, which must be a multiple of `$800`. The whole file is therefore `$800` plus this.
`$020`	4	Unused address	Unknown, usually 0; the page suggests an optional overlay that is not loaded automatically.
`$024`	4	Unused size	Unknown, usually 0.
`$028`	4	Memory fill start address	The start of a RAM area to fill before the program starts; usually 0.
`$02C`	4	Memory fill size	The number of bytes to fill; 0 for none.
`$030`	4	Initial SP and FP base	The stack pointer (R29) and frame pointer (R30) start at this address plus the offset below. Usually `$801FFFF0`; 0 for none.
`$034`	4	Initial SP and FP offset	Added to the base; usually 0.
`$038`	20	Reserved	Reserved for the BIOS function A(43h), Exec; should be zero in the file.
`$04C`	to `$7FF`	Region text	One of the three texts below, then zeros up to `$7FF`. The BIOS does not check it and boots without it; it is often all zeros in homebrew.
`$800`	size	Program	The code and data, loaded to the destination address and up.

Region text	Region
`Sony Computer Entertainment Inc. for Japan area`	Japan (NTSC)
`Sony Computer Entertainment Inc. for Europe area`	Europe (PAL)
`Sony Computer Entertainment Inc. for North America area`	North America (NTSC)

The header carries no relocation information: the program must run at the destination address.

What the viewer checks

The size is a multiple of $800, and the file is exactly $800 plus the size; a longer file has bytes the BIOS does not load, a shorter one is truncated.
The initial PC is inside the loaded program and word aligned.
The destination is not in the first 64 KiB of RAM and the program ends within the 2 MiB of RAM.
The bytes the format says are zero ($008–$00F, the end of the header) are.

Not described

Known, not yet covered

Reference: what the unused address and size at $020 and $024 are for (the source calls them unknown), how the BIOS fills the reserved words at $038 when it executes a file, and the layout of the “PS-X EXR” relocatable variant.
Viewer: the viewer only reports problems; whether the BIOS refuses a file for any of them is not described (the source says only that the region text is not verified).

PS-X EXE File Format – Header

Introduction

Fields

What the viewer checks

Not described

Known, not yet covered

Links

Reference