visrealm/vremutms9918

TMS9918A emulator library (C99)

vremutms9918's Issues

Write then read of VDP vram should return previously written value

According to this hardware deep-dive VDP hardware uses a read-ahead buffer for speed and other reasons. This results in some interesting behavior if you mix reads and writes. Most obviously:

1. write a value to the data port (mode=0)
2. read from the data port (mode=0)

Due to the read-ahead buffer, you will get back the written value, you will not get the value advanced by the pointer. There are other behavioral oddities here as well regarding how read and write deals with the vram pointer.

When reading:

1. value in read-ahead buffer is returned
2. read-ahead buffer is updated with value pointed at by current vram address
3. vram address is incremented

When writing:

1. vram pointed at by vram address is updated with new value
2. read-ahead buffer is set to new value as well
3. vram address is incremented

I have verified this with my own hardware but worth checking on yours as well. I'm using TMS9128.

VRAM address should only be set on successful register write pair

Current implementation can inadvertently erase the current vram address if you:

1. write a legitimate vram address to the VDP so it's pointing somewhere expected
2. write a value to register port (mode=1)
3. read status register (mode=1)
4. read data port (mode=0)

VDP has now overwritten lower byte of vram address. According to this hardware deep-dive, changes are only committed on successful pairs.