The fourth letter of the "SCEx" string contains region information: "SCEI"(Japan/NTSC), "SCEA" (America/NTSC), "SCEE" (Europe/PAL). The "SCEx" string isdisplayed in the intro, and the PSX refuses to boot if it doesn't match up forthe local region.With a modchip installed, the same response is sent for Mode1 and Audio disks(except for Audio disks with very short TOCs (eg. singles) because SCEX readingis aborted immediately after reading all TOC entries on Audio disks); whetherit is Audio or Mode1 can be checked by examining Subchannel Q ADR/Control.Bit6(eg. via command 19h,60h,50h,00h).Yaroze does return "SCEA" for SCEA discs, but, for SCEI,SCEE,SCEW discs it doesreturn four ASCII spaces (20h).
the Logo contains a header, polygons, vertices and normals for the "PS" logo(which is displayed when booting from CDROM). Some BIOS versions are comparingthese 3278h bytes against an identical copy in ROM, and refuse to boot if thedata isn't 1:1 the same:- US/ASIA BIOS always accepts changed logos.- PAL BIOS accepts changed logos up to v3.0E (and refuses in v4.0E and up).- JP BIOS never accepts changed logos (and/or changed license strings?).Note: PAL BIOS with "region-patch-modchip" does behave same as US/ASIA BIOS.
Once when the PSX has recognized a disk with the "SCEx" signal, it'll besatisfied until a new disk is inserted, which is sensed by the SHELL_OPENswitch. When having that switch blocked, it is possible to insert a CDR withoutthe PSX noticing that the disk was changed.Additionally, the trick requires some boot software that stops the drive motor(so the new disk can be inserted, despite of the PSX thinking that the drivedoor is still closed), and that does then start the boot executable on the newdisk.The boot software can be stored on a special boot-disk (that do have the "SCEx"string on it). Alternately, a regular PSX game disk could be used, with theboot software stored somewhere else (eg. on Expansion ROM, or BIOS ROMreplacement, or Memory Card). 2b1af7f3a8