My first program was to test some concepts (File: SerialTest.txt). Can I read a source program and output binary? Actually, for ease of debugging, the "object code" from 1st pass will be readable hex.

This failed. A bug in the simulated computer of the badge, means that if there are characters queued in the FIFO input buffer, and you write one character, then subsequent reads return that written character. It works if the FIFO is empty. That killed the idea of simply spooling the source file to the computer from a host laptop and capturing the output (ie full duplex, no local echo, in a terminal program).

There was also a 2nd bug in the Serial implementation - it could not use the same pins as the loader does, Setting bit 0 WrFlag didnt have an effect, meaning the RX pin switches position between program and loading.

Possible workarounds for the duplex

1. Use some hardware flow control. It may need a non-standard USB/Serial adaptor. [Not tested, yet. It may not be "fast enough" to stop characters - USB/Serial tend to batch data]
2. The external device (a host laptop, typically) only sends one line, then waits for a response.
3. Don't use the Serial UART for output but implement a bit-bang serial on one of the 4 output lines.

Initial code will use (2)