The undefined 'x' appear to be from the upper byte when doing a 16-bit pixel read but more testing needs to be done to confirm this.
* setting a partial vram write address is ok. e.g. set r0,r1,r2 to &80000, now set r1 to 1. Write will go to 000001. Now set r2 to 2. Write will go to 000201. Now set R3 to 3. Write will go to 030201. There doesn't appear to be a write buffer.
* v9990 seems to have a 1 byte read buffer. Setting a partial address and the reading vram will return the data from the previous full address set for the first read, but further reads returns correct values. This is only true of r3 and r4. e.g. set r3,r4,r5 to &80000. Now set r3 to 1. First read will come from &000000, second and subsequent reads now come from &000001. Now set r4 to 2. First read comes from &000001, second and subsequent reads come from &000201. Now set r5 to 3. First read comes from &030201, subsequent reads come from &030201.
[[Category:Hardware]]