Questions tagged [mos-650x]

For questions on the MOS Technology 650x family of processors. For questions about the 6502 use the "6502" tag.

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10 votes
5 answers

Why does MOS 6502 require an external clock if it has an internal oscillator?

I have a trouble understanding how the MOS 6502 clock works. Possibly due to my extremely low knowledge regarding electronics in general. According to the Wikipedia: MOS would introduce two ...
  • 201
24 votes
1 answer

Who ended up owning Commodore's semiconductor IP?

The question of ownership of Amiga-related intellectual property post-Commodore is famously difficult to answer. My question is: does anyone "own" Commodore's semiconductor IP now, and if so,...
8 votes
3 answers

Were any Apple 1s sold with non-ROR 6502s?

The first production runs of the MOS 6502, in mid-1975, had a bug in the ROR instruction that caused MOS to "remove" it from the instruction set by omitting it in the intial documentation. ...
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21 votes
1 answer

MOS 8502, just a 6510B?

I'm looking over what little I can find on the 8502, and from what I can see it appears to be a 2 MHz version of the 6510 with an extra I/O pin. Much ado about the speed, but the Atari's 6502B's were ...
4 votes
1 answer

65C816: Inputs TTL-compatible?

New 65C816 datasheets say that Vih (for the data bus in particular) is 0.8Vdd, i.e. not TTL-compaible. Old data sheets (for example, the one published in Apple IIgs Hardware Reference) say that at 5V ...
10 votes
2 answers

6510 I/O port initialisation

TL;DR: With what values are the I/O port registers (locations $00 and $01) on the 6510 initialised when it is powered on? I'm writing a Commodore 64 emulation. The Commodore 64 uses a 6510 ...
  • 10.9k
6 votes
1 answer

Commodore-Motorola lawsuit in 1984

According to Commodore: The Amiga Years, the reason the Commodore UNIX machine project in the early 80s used the Z8000 rather than the 68000 was an ongoing lawsuit with Motorola: With the Commodore-...
  • 53.4k
17 votes
2 answers

6502 branch offset calculation

This question expands on How does the 6502 implement its branch instructions? I'm working on a cycle accurate VHDL implementation on an FPGA. I have much of the program logic already written, but I ...
  • 173
18 votes
1 answer

How did the Atari 2600's 6507 handle zero page and stack with only 128 bytes of RAM?

The Atari 2600 used a cut-down version of the 6502 called the 6507. The 6507 was cost reduced by not supporting interrupts and (more importantly) having fewer address lines resulting in that it only ...
  • 15k
10 votes
2 answers

How does the 6502 implement its branch instructions?

The 6502's decode ROM takes an input which tells it when to start the next instruction. In the case of something like an INX or LDA #7F, it is obvious that once the output from the ALU has been ...
32 votes
6 answers

Why did Commodore sell CPUs to its competitors?

Many of the most popular personal computers and video game consoles of the 1970s and 1980s, including those made by Commodore, Apple, and Atari, used the 6502 CPU (or some close relative, such as the ...
  • 6,722
19 votes
2 answers

Is it possible to procedurally determine the number of cycles a particular instruction takes on a 6502?

Most emulators store the number of cycles a particular instruction takes in an array, and then adds any conditional cycles if needed (when crossing page boundaries, for example). I'm wondering if ...
  • 393
27 votes
4 answers

Use of undocumented opcodes

I figure this is fitting for Retrocomputing.SE because CPUs like the 6502 and Z80 and the PDP-8, and probably others, have undocumented instructions and will happily execute them, in contrast with ...
14 votes
2 answers

What is a use case for the JAM instruction on the MOS Technology 650x family of processors?

I was reading about undocumented opcodes for the 650x family of processors and discovered JAM, an instruction "which simply causes the CPU to freeze, requiring a hardware reset or power cycle to ...
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