Questions tagged [6502]
For 6502 series of processors, including hardware and assembly language questions. Use with [hardware] for the hardware interface in particular.
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Was BCD a limiting factor on 6502 speed?
In a comment on https://retrocomputing.stackexchange.com/a/30215/4274 supercat made this interesting remark:
I think the critical paths with regard to cycle rate occurred while doing BCD math. A ...
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Why didn't Commodore run the 6502 at 2 MHz? [closed]
The Atari 800, in 1979, ran the 6502 at 1.79 MHz. That rounds to 2, and was presumably a 6502A rated for 2 MHz, underclocked.
Why did Commodore keep running their 6502 computers at 1 MHz, even the 64 ...
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Did the NES CPU save die area by omitting BCD?
The NES CPU was a copy of the 6502 with the BCD circuitry removed. As I understand it, this modification was motivated by a theory that BCD was the only part of the 6502 that was actually patented, so ...
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What process node enabled the 6502 to reach 2 MHz? [duplicate]
The original version of the 6502 was rated for 1 MHz, but before the end of the decade, the CPU was available in a 2 MHz version. Presumably what changed was Dennard scaling: make the transistors ...
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NES vs SMS ROM speed
The NES used a 6502-derived CPU at 1.79 MHz.
The Sega Master System used a Z80 at 3.58 MHz.
How did they differ in terms of what speed grade ROM they required in their respective cartridges? I know it'...
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Why did Acorn use the 6502?
It is an interesting quirk of the British microcomputer industry, that the main vendor of cheap microcomputers, Sinclair, used the better, more expensive CPU (Z80), whereas the main vendor of better, ...
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Why does the BRK instruction set the B flag?
On the 6502, the brk instruction is a software interrupt. Like any other interrupt, it pushes the status word to the stack and then the program counter, before transferring control to an interrupt ...
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What elements of the MOS 6502 did it copy from the Motorola 6800?
It is not uncommon for microprocessor system designers to copy elements of
design from other microprocessor systems; the Z80, extensively copying
design elements of the 8080, is a notable example.
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What was the 6502's reset behaviour when the RDY pin was low
How does an original 6502 behave if the ready pin is being held during a reset/power-on event?
Would it pause CPU start-up entirely, delaying when some or all of the 6 internal cycles are executed? (...
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Why is the addressing mode for BRK defined as "stack" in the W65C02S datasheet?
I most online documentation, I find the addressing mode for BRK to be "implied", which is logical. In the W65C02S datasheet however, it is set as "stack":
Is there some reasoning ...
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Is there an standardised 6502 assembly syntax?
I am working on a Flex/Bison based 6502 assembler (for fun and learning) and looking around for a "standard" syntax for the assembly language. Opcodes and addressing are more or less ...
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ProDOS BASIC.SYSTEM overwrites RNDL ($4E) - how to get random numbers
I am writing an application that uses random numbers. The plan is to use RNDL/RNDH ($4E) to initialize a pseudo-random number generator. Unfortunately, I keep getting the same sequence. Tonight, I ...
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Did any 8-bit device ever merge a CPU core?
Looking at https://upload.wikimedia.org/wikipedia/commons/c/c1/Commodore-64-1541-Floppy-Drive-04.jpg I started thinking the following:
There are too many chips in that drive. It is crying out for a ...
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Synchronising FOR - NEXT routines in C64 basic [closed]
I am writing a program which has an ASCII background, animating character moving to random locations, and finally adding music
It's my first ever C64 attempt and I want to finish it in BASIC before ...
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Help understanding TAIPAN source code for the Apple II
I am trying to run the source code found at https://taipangame.com/BASIC.txt on Epple 2 (with Apple II+ roms) to eventually run on my own Apple II+, but the program keeps crashing.
First, I used a ...
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