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For developers wanting to explore FCM (full-colour mode), it can be quite a daunting journey initially, so here’s some pointers along with a ‘experimentation’ walkthrough that will let you peek and poke your way to a better understanding of FCM (smile)

The gist of FCM/NCM modes

For FCM, each character on the screen is represented by 4 bytes (32-bits worth of information)

  • Two of those bytes are in screen memory (where we'd traditionally put our characters. e.g. at $0800)

  • Two of those bytes are in colour memory (e.g. at $ff80000)

For NCM (nibble colour mode), each two screen characters are defined by these 32-bits)

From here-on in, I’ll refer to the concept of “two of those bytes” as “byte-pairs”.

Where to read about it in the docs?

The section of interest is in "Appendix M >> New Modes >> Displaying more than 256 unique characters via "Super-Extended Attribute Mode"

Of particular interest is these two tables. They describe how the bit-fields within the screen byte-pair and the colour byte-pair are defined.

The major gotchya that trips up newcomers is that the definition of these bit-fields varies depending on whether a certain bit within the colour byte-pair is set or cleared, the GOTOX bit.

That’s why there are two tables:

  • one table for when this GOTOX bit is cleared

  • the other table for when this GOTOX bit is set

FCM Experimentation via the UART monitor

In this section, I share my experimentation notes, all of my past attempts to peek and poke various registers in order to learn how these byte-pairs (and the bit-fields within them worked).

My notes feature a lot of screenshots, and follow this approach:

  • Try this, observe that…

  • Try this, observe that…

  • rinse/repeat

…so I feel it will be relatively easy for someone to simply repeat the steps I tried and gain similar understanding and insights.

How to access the UART monitor on a real MEGA65

If you’re working on real hardware, you have two options:

  • “OK OPTION”
    - You can press MEGA+TAB keys to reveal the matrix debugger and type monitor commands within there

    • PROS: in-built option all owners have available to them from the get-go

    • CONS: The monitor is shown as an overlay over the original screen, so you will be jumping in-and-out after each command you type, and this may limit your ability to view the impact of the change in real-time.

  • “BETTER OPTION”
    - If you have a JTAG/serial adapter connected to your MEGA65, you then debug via USB-serial on your PC using either a serial terminal program like putty, or you could use a fully-fledged debugging tool like m65dbg.

How to access the UART monitor in XEMU

In xemu:

  • right click for context menu

  • Go to "Debug/Advanced >> Start umon on :4510"

  • Then in a console (or in a tool like putty), telnet to localhost on port 4510. E.g.:
    telnet localhost 4510

  • You can then type monitor commands within this telnet session

How to use the UART monitor

For ‘peek’ing, you can use the 'm' command:

m<addr>

For ‘poke’ing, you can use the 's' command (it can take multiple values if you want to write several bytes to successive addresses):

s<addr> <val1> [<val2> <val3> ...]

So, where can I find your FCM experimentation notes then?

My notes are available as a google doc here:

Gurce's "FCM Experimentation" notes

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