Maker and classic computing fanatic Anders Nielsen has introduced a bit of pc historical past to life by constructing a contemporary incarnation of Steve Wozniak’s single-stepping circuit for the MOS Expertise 6502 eight-bit microprocessor — and, in doing so, has put to mattress rumors that it could not probably work as-designed.
“In case you’ve ever performed round with the 6502 microprocessor present in a ton of previous computer systems, like those from Atari, Commodore, Apple, and Nintendo, you would possibly’ve heard that you simply completely should get the CMOS model that got here out within the Eighties as a result of the unique 6502 from 1975 cannot be run one cycle at a time, making debugging more durable,” Nielsen explains of the parable he got down to bust. “I am glad to let you know that you have been mislead and it is not solely potential to single cycle the unique 6502 — it is an deliberately built-in characteristic with it is personal devoted pin.”
Properly-read Apple followers will know that co-founder Steve Wozniak included a circuit for single-stepping a MOS 6502 processor — manually clocking it in order that it will solely run an instruction when a button was pushed, permitting packages to be debugged on the lowest potential stage — within the handbook for the Apple-1, the corporate’s first private pc as-launched in 1976.
In more moderen years, although, the performance of that circuit design was introduced into query with claims that solely the later complementary metal-oxide semiconductor (CMOS) model of the 6502 would work, and that it will fail on the unique N-type MOS (NMOS) model of the chip.
“The confusion on the matter stems from the truth that the [NMOS] 6502 has dynamic registers and the newer [CMOS] 65C02 is a totally static design,” Niesel explains. “Meaning the 65C02 will not lose the contents of its registers for those who cease the clock, whereas the unique 6502 will if run it slower than about 10kHz.”
Nielsen’s replica board is near-identical to Wozniak’s unique schematic, bar the addition of pull-up resistors. (📷: Anders Nielsen)
Whereas Nielsen discovered nothing mistaken with the idea of Wozniak’s design, truly getting it to work within the twenty first century was just a little more difficult — however solely barely. “We have to replace the schematics a bit by placing pull-up resistors on a lot of the inputs,” Nielsen explains. “Aside from that, I needed to take a very good lengthy look within the junk drawer to search out some acceptable switches.”
A video on Nielsen’s recreation of the single-stepping circuit is embedded above and accessible on his YouTube channel, whereas design recordsdata for the board have been printed to GitHub below the reciprocal GNU Normal Public License 3; Nielsen can also be promoting assembled model of the board on iMania.dk at 46.88 DKK (round $9.60) plus delivery.