Accumulator Addressing

[chysn]

Back when I was a kid, using available assemblers like HESMon, I learned that the accumulator addressing syntax was implied. So

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LDA #$80
LSR

Nowadays, when I look at the Commodore VIC-20 Programmer's Reference Guide, Lance Leventhal's books, and VIC Revealed, all these resources specify the Accumulator addressing mode explicitly, like this

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LDA #$80
LSR A

Okay, two ways to do the same thing. But I'm finding that some modern assemblers like xa and Acme complain when the Accumulator is explicit (the second syntax), while mass:werk's online assembler complains about the implicit syntax. Personally, I prefer the implicit syntax because that's what I used when I was a kid. But shouldn't assemblers treat both the same?

This is an issue for me now because I'm starting to put my 6502 code on Github, and I'd like to find some degree of portability.

[Mike]

Both syntaxes are found in the wild in sources.

The MOS documents specify explicit accumulator addressing. A lot of monitors of that time instead used implicit accumulator addressing as that can be implemented with less code. Today's tools either (again) leave out explicit accumulator addressing, or enable/disable this as command-line switch. The rationale being, that explicit accumulator addressing is essentially redundant, or are least a possible source of confusion: is "A" maybe a symbol, or even the address $0A in implicit hex notation? You cannot have both types in one source as that would introduce said ambiguity.

In practice, you will always have to infer from another persons source whether the used assembler wants implicit or explicit accumulator addressing. For own source, just choose the tool which suits you best.

My toolchain contains a monitor which outputs implicit accumulator addressing, and an assembler, which wants explicit accumulator addressing; so I am very much aware of that issue and still can live with it.

[funkheld]

Hi good afternoon.

The assembler instruction LSR shifts the eight bits of the accumulator one place to the right and not 2...3...4..

ok is :
LDA #$10
LSR



or 3 Bits:
LDA #$10
LSR
LDA #$10
LSR
LDA #$10
LSR

greeting

[groepaz]

think again

[chysn]

My toolchain contains a monitor which outputs implicit accumulator addressing, and an assembler, which wants explicit accumulator addressing;

I'm sure your monitor is your own Minimon May I ask what your assembler is?

so I am very much aware of that issue and still can life with it.

Sure, in the human panoply of minor grievances and annoyances, this is hardly worth mentioning. In fact, the diversity of assemblers for the 6502 in the year 2020 is nice to see.

or 3 Bits:
LDA #$10
LSR
LDA #$10
LSR
LDA #$10
LSR

Be careful here. You're resetting your accumulator each time, so this won't do what you seem to expect.

[Mike]

I'm sure your monitor is your own Minimon

Yes, this is MINIMON but I also still use TEDMON (the 264 variant, mainly when a program occupies address ranges overlaid by ROM or I/O on the VIC-20 and I want to relocate it).

May I ask what your assembler is?

I use the inline assembler of BBC BASIC. To be more specific, it's the one of HI BASIC which runs on the 2nd processor Tube.

One of the nicer things is, you have the full expression parser available to calculate addresses.

With the program MG BROWSE, I included source for the machine code part. There, you might take a look how 'typical' source for that assembler looks like (and you find prominent examples of explicit accumulator addressing with quite some 'ROL A' instructions in the TDraw routine).

Greetings,

Michael