ReCode /

It is only natural for people to think with nostalgia about the good 'ol days: "oh man, back in the day things were so simple!" Objectively speaking, obviously, the "good 'ol days" sucked: that simplicity is only skin deep.

Yes, a telephone was much easier to use in the 70s, but then again it didn't do that much, did it? Could you imagine having to memorize the phone numbers of everyone you need to text or call? Right, that would have sucked. How about having to fast forward in order to get to the next song on a compact cassette? Ouch.

Yet, some things from the past are worth admiring. To me, a steam engine is prettier than the most advanced internal combustion engines of today. Not really practical, yet pretty.

But I'm a programmer, and a lot of the old stuff I admire is software. And while I have (if only once) programmed a computer with punchcards, those were already antiques when I was in school; the computer I grew up programming was the Apple ][.

So here we go: at the risk of wasting my time raving about something most programmers today can't appreciate, I'll try to explain but just a tiny bit of Wozniak's brilliancy:

The Apple ][ floppy disk controller bootstrapping routine

Some background: on physical magnetic media, you can't "just record data". Instead, 1 is encoded as a magnetic polarity transition (also known as flux reversal), and 0 is encoded as, well, lack of transition. Basically, for each bit there is a limited time window in which flux reversal is expected to occur. If it occurs, the bit is 1, otherwise it's 0. The problem is that the time window is so small that there is a limit on the accuracy with which the data window length can be measured. So, while detecting a series of ones is not a problem, it's difficult to know how many zeroes are encoded in a measured period of lack of flux reversal.

Wozniak's original hardware design imposed two constraints on the raw data:

• between any two 1-bits, there can be no more than one 0-bit, and
• each 8-bit byte must start with a 1-bit

However, using simple FM encoding "wastes" 4 bits per raw byte, which would allow only 10 256-byte sectors per track to be recorded. Instead, he devised a more complex encoding scheme that was based on the fact that there are 34 8-bit numbers which have the top bit set and no two 0-bits in a row. That way, 13 sectors per track could be recorded. Later on the hardware design of the floppy disk controller was modified to allow no more than one occurrence of two consecutive 0-bits in a byte, which lead to different encoding called 6&2; it allowed 16 sectors per track.

A conventional floppy disk controller design would put all this bit twiddling in silicone which just DMAs the decoded bytes in memory -- but it would cost more in hardware. So what is a Wozniak to do? Obviously: screw hardware, do everything in software.

That would have been impressive enough on a 6502 CPU, but even more impressive is what is required of the bootstrapping routine. It must:

• generate the 6&2 decoding table in RAM
• seek the disk drive to track zero
• look for the beginning marker of sector zero and read the raw data in RAM
• decode the data and jump into it

...all in no more than 256 bytes of code, which is the addressing limit for the ROM on Apple ][ expansion cards.

By the way, there are no timers on the Apple ][, all timing is based on the CPU clock, e.g. you'd know that an INX instruction takes 2 microseconds to execute. The 6502 has three 8-bit registers: an accumulator A which can add, subtract, shift, rotate, and, or, xor, etc., and two 8-bit index registers X and Y which can do none of those operations. :)

Here are some pictures I found of Bulgarian-made Apple ][ clones: the first two are of Pravetz-82 from the late 70s/early 80s, the third picture is of Pravetz-8C from the late 80s/early 90s. These Bulgarian computers were compatible with Apple ][, but their evolution didn't entirely follow the evolution of Apple ][, e.g. some models didn't have an exact original.

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