Don’t you love it when new technical details come to light?
In an effort to comprehensively document everything there is to possibly know about Virtual Boy (yes, I’m working on something big), I set my sights on the CPU’s system registers. For the uninitiated, the Virtual Boy’s CPU has not only 32 general-purpose program registers, but 10 system registers containing things like status flags, hardware breakpoint configuration, the processor’s ID, etc.
System registers can be accessed by the program: they can be written to with the LDSR instruction, and read from using the STSR instruction. Those instructions contain a 5-bit register ID field, meaning 32 different IDs, although V810 only defines ten system registers, as follows:
0 EIPC Exception/Interrupt PC 1 EIPSW Exception/Interrup PSW 2 FEPC Duplexed Exception PC 3 FEPSW Duplexed Exception PSW 4 ECR Exception Cause Register 5 PSW Program Status Word 6 PIR Processor ID Register 7 TKCW TasK Control Word 8-23 (reserved) 24 CHCW CacHe Control Word 25 ADTRE ADdress Trap Register for Execution 26-31 (reserved)
What I was doing was seeing what would happen if you accessed those “reserved” IDs. I mean, what if something happens?
Something happens.
System registers 8-23 and 26-28 probably don’t map to anything. They read back as all zeroes, and cannot be written to. System registers 29-31, on the other hand, exist, and they’re not any of the V810 system registers.
If anyone is able, please look into these to see if we can find additional hardware functionality as of yet unknown to the VB community.
System register 29 can store and load all 32 bits, and it is the only system register capable of doing so (even FEPSW and ADTRE mask off a few bits). Its intended purpose is unkown, and it’s not locked down to an execution address range. For now, it looks like a feasible target for storing a global value not in system WRAM.
System register 30 is read-only and has a value of 0x00000004. This is probably a version ID for the NVC chip, but there’s no way to test that hypothesis.
System register 31 masks off the upper 30 bits, but bit 0 can be written. It’s possible there’s some kind of hardware feature that is enabled and disabled by this register, but I haven’t the foggiest what it is. It’s probably related to the CPU directly; otherwise the flag would be somewhere in the hardware I/O bus address ranges instead.
So again, if anyone can put some investigation into these undocumented system registers, we just might be able to uncover something about Virtual Boy that’s never been uncovered before.
I’ve attached a ROM to test this. Up and Down on the left D-Pad change the current system register, and the A button writes 0xFFFFFFFF to that register. Once that happens, a third line of text appears showing the value that was in the system register immediately after writing to it.
