This is a small tandem processor board which is to be plugged into a PC's 8-bit ISA-slot.
The board contains a 6502A CPU. The clock (4.77 MHz) provided by the ISA-slot is divided by 2. So the 6502 is driven by 2.38 MHz.
The memory is subdivided into 8 pages of 8kB each by using a 74138 to generate those 8 chip select signals from the 6502's address lines A13-A15. The PC can halt the 6502 and decouple it from it's memory. So the PC has full access over the 6502's 64kB memory address space. That's the way the PC can transfer programs/data to/from the tandem board.
The PC can trigger the 6502's interrupts and the 6502 on the other hand can trigger the PC's interrupts. So they are able to send signals to each other to initiate a information interchange.
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