Refer to microcode and source code. 

This component has the following capabilities:

• Accept ASCII characters on its 8-bit write-only port with simple handshake logic (it can interface with an UART to accept serial stream from host computer)
• Interpret incoming character stream as records in Intel .hex file format
• If the record received is valid (no bad characters, right number of bytes, good checksum), write the record to memory
• Memory interface is 16-bit address, 8-bit data, and using DMA logic similar to Z80 (BUSREQ - BUSACK)
• Signals error on a dedicated pin
• Provides a debug serial port to trace execution of the internal microcode