I/O Map
The following table lists the main I/O definitions of the VP4801. This is likely not 100% correct and missing information.
| EF1 | If both EF1 and EF2 are 0 the VP4801 will clear RAM for a cold startup |
| EF2 | If both EF1 and EF2 are 0 the VP4801 will clear RAM for a cold startup |
| Address | |
| 0xC400 | Maybe TMS99532 modem output? |
| 0xCC01, 0xCC02, 0xCC04 | ??? On startup 0x03 is written on 0xCC01, 0x38 on 0xCC02 and 0x41 on 0xCC04 |
| 0xCE02 | Maybe TMS99532 modem input? bit 1&2: if one of these is 0 the VP4801 will go into some kind of online mode. Maybe if modem data is detected via DCD? bit 3: if 0, the VP4801 FW will lock until it becomes 1. Maybe modem status or TMS99532 oscilator? bit 5: if 0, COUPLER text is stored in RAM at 0x8120 (Accoustic Coupler in the TMS99532?). if 1 and 0x8100 bit 7 is 1 then PHONE text is stored in RAM at 0x8120. If 0x8100 bit 7 is 0, then spaces are stores in RAM at 0x8120. |
| Address | SCN2672 Programmable Video Timing Controller |
| 0xC000 | Read Interrupt Register Write Initialization Register |
| 0xC001 | Read Status Register Write command code |
| 0xC002 | Read & write Screen Start Register (LSB) |
| 0xC003 | Read & write Screen Start Register (MSB - bit 0 to 5) |
| 0xC004 | Read & write Cursor Address Register (LSB) |
| 0xC005 | Read & write Cursor Address Register (MSB - bit 0 to 5) |
| 0xC006 | Write Pointer Address Register (LSB) |
| 0xC007 | Write Pointer Address Register (MSB - bit 0 to 5) |
| 0xC008 | Read Light Pen Address Register (LSB) |
| 0xC009 | Read Light Pen Address Register (MSB - bit 0 to 5) |
| 0xC800 | Read & write Bus Interface Latch |
| Address | SCN2671 Programmable Keyboard and Communication Controller |
| 0xC200 | Read Interrupt Status Register Write Reset Command |
| 0xC201 | Read & write Communications Mode Register |
| 0xC202 | Read Receiver Holding Register Write Transmit Holding Register |
| 0xC203 | Read Communications Status Register Write Baud Rate Mode Register |
| 0xC204 | Read & write Interrupt Mask Register |
| 0xC205 | Read & write Keyboard Mode Register |
| 0xC206 | Read Keyboard Holding Register |
| 0xC207 | Read Keyboard Status Register Write Miscellaneous Command |