* If reset is held: 3,4,7,8,9,10,11,12,13,14 and 15 reads databus, port 1 reads 0, port 5 and 6 read status,
=== VRAM ===
* In terms of physical addresses, if you consider physical address 0-&3ffff to be VRAM0 and physical address &40000-&7ffff to be VRAM1, then in P1 mode, the physical address equals logical address and in bitmap modes, every even address maps to &0-&3ffff (ever even is VRAM0) and every odd address maps to &40000-&7ffff (VRAM 1). This is described in the PDF.
* 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 address 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.
* If vram is set to 128KB a byte is written and then vram is set to 512KB you can read the same byte in multiple locations. The written data is effectively mirrored in the address space.
== Command Engine ==