Thank you! It just amazes me the code I've been looking at even works with all the cruft and inefficient coding I've dug out so far.

If the only interrupt I'm working with is driven by the NMI pin on the CPU, are any SEI / CLI instructions therefore meaningless?

Or are there internal IRQs within the processor that are not otherwise addressed by the user's code?

I'm curious as to how your .receieveBits routine works.

I have some code I'm working on that also involves converting a serial stream into bytes:


lda piaPortA
ror A
rol presentSIByte
; piaPortA is port A on a MC6821 PIA
; roll bit 0 to carry, then into current byte being received

(test status ...

There we go. Looks like that was the last bit that needed cleaning.

Now that I got the last of the spaghetti picked out and the one 'test byte' ($B9) documented in regards to which conditionals are tripped by which values, I can focus on the interrupt section.

I guess from seeing CMP and BEQ / BNE ...

I guess I'm a bit confused on the proper use of AND in conjunction with BEQ / BNE versus in conjunction with CMP.

So, if I do the AND with 6, leaving 0, then the Z flag goes up, at which point BEQ kicks in, right?

Whereas my thinking of BEQ has centered on whether the remainder after being ANDed ...

Barry: I grabbed the wrong bit of code before posting (it was a too-quick copy / clean / paste from the original 68xx code). I made a correction that should help.

So, I've got the bit of code I've been working on mostly cleaned up and reorganized to flow more efficiently and logically, but there's still one bit that has me scratching my head.

Perhaps it's a bit of leftover cruft that was never tidied up (wouldn't be surprised, given the inefficient use of ...

cjs: $D8 / $D9 is a fixed offset with value $0083 that gets nudged up to $0084, then back.

This is just one of many bits of 'cruft' in the code... given certain aspects of the original unit's hardware design (it has a #$?!?!! hot chassis - even when the power switch is OFF) I'm not entirely ...

Still picking at the code to see how it works, particularly the interrupt code, and I wound up making a chart by RAM address to see which locations are read and written and where in the code they are called on.

I just now noticed that the TST $00CC conditional when called, drops down to another ...

As for using BIT to wait for one line to change state, that's actually not a bad idea. Will have to see if that would work better than the existing approach.

Anyway, coming back to the original part of my post, for the post-test relay check routine I did end up using BIT rather than LSR paired with ...

This successive bit-shifting is done in several places in the code.

The most common use is to interpret the relay status nybble to determine whether the four relays opened or closed as directed, and to set the pass / fail conditions as appropriate.

It is also used within the main loop; at various ...

Currently working in 6801 assembly, yes, so if / when this gets translated over to 6502, the B accumulator in this example would end up being substituted with the Y index.

Here's a snippet showing what I mean about using the carry bit to evaluate a number and if I understand BIT based on what you guys say:


Relay status nybble: 0 = contacts closed, 1 = contacts open

Existing code:

LB_ECC0 LDA A $C3 // fetch status nybble
LSR A // shift Relay A bit into carry
BCS ...

So, I've never really been clear on the BIT instruction, as I so seldom encountered this opcode when poking around in assembly language back in my C64 days.

Are there any real-world code examples that would illustrate how this works?

I have at hand some code that could stand a slight rewrite, and ...

Long story short, the code pertaining to $D8 can pretty much go at this point, but the TST $CC stays.

I have found a fair number of 'make it work' kludges and programming inconsistencies so far, and $D8 and $DE go into this pile as well. $D8 looks to be an offset that apparently allowed the main ...