V9990

• In the following "databus" means that the v9990 doesn't assert the bus, it doesn't provide data, so the value read is "floating-bus" or the value on the z80 databus at the time. More commonly on CPC it's &ff, but there are some configurations of CPC where this differs.

• Reading from the Kanji ROM ports without a Kanji ROM returns databus values

• Reading from the write only ports (or unused ports) returns databus values.

• Palette red data has a mask of &9F (bits red and color key bit), blue and green have a mask of 0x01f.

e.g.

LD BC,&FF64
LD A,14
OUT (C),C
LD BC,&FF63
LD A,0
OUT (C),A

LD BC,&FF61
LD A,&FF ;; <- reading this component back returns &9F
OUT (C),A 
LD A,&FF ;; <- reading this component back returns &1F
OUT (C),A 
LD A,&FF ;; <- reading this component back returns &1F
OUT (C),A 

• Bits 1,0 of R14 define a ternary counter. If you set this to 3 then reads and writes are ignored.

e.g.

e.g.

LD BC,&FF64
LD A,14
OUT (C),C
LD BC,&FF63
LD A,3
OUT (C),A

LD BC,&FF61
LD A,&FF 
OUT (C),A ;; write is not done.

LD BC,&FF64
LD A,14
OUT (C),C
LD BC,&FF63
LD A,3
OUT (C),A

LD BC,&FF61
IN A,(C)  ;; read returns last value written to port

When the ternary counter is 2, and auto-increment is enabled, the ternary counter will wrap to 0 and the palette index will increment.

e.g.

LD BC,&FF64
LD A,14
OUT (C),C
LD BC,&FF63
LD A,2
OUT (C),A

LD BC,&FF61
LD A,&FF 
OUT (C),A 

palette index will be 1, and ternary counter is 0.

• If a command transfers data, and it requests data to be written to the command data port, then you can't use a read of this port to clear the data request. It must be a write. Similarly, if a command requests data to be read then you can use a write to clear the data request.

• If you read from a write-only register you will see data-bus value because the V9990 doesn't assert data on the bus.

• Some registers have additional bits which are not documented. The mask describes which bits are read/write and which are unchanged.

Where the mask has a '1' bit, this bit is read/write. Where the mask has a '0' bit this bit remains at 0 and can't be changed.

  - Register 7 (SCREEN MODE): Mask is &FF.
  - Register 9 (INTERRUPT READ/WRITE): Mask is &87
  - Register 15 (BACK DROP COLOR): Mask is &FF 
  - Register 22 (SCROLL CONTROL): Mask is &C1
  - Register 25 (SPRITE PATTERN GENERATOR TABLE BASE ADDRESS): Mask is &CF
  - Register 26 (LCD CONTROL): Mask is &FF
  - Register 27 (PRIORITY CONTROL): Mask is &FF