Comment by zabzonk
9 days ago
I've written an Intel 8080 emulator that was portable between Dec10/VAX/IBM VM CMS. That was easy - the 8080 can be done quite simply with a 256 value switch - I did mine in FORTRAN77.
Writing a BASIC interpreter, with floating point, is much harder. Gates, Allen and other collaborators BASIC was pretty damned good.
>Writing a BASIC interpreter, with floating point, is much harder. Gates, Allen and other collaborators BASIC was pretty damned good.
The floating point routines are Monte Davidoff's work. But yes, Gates and Allen writing Altair BASIC on the Harvard PDP-10 without ever actually seeing a real Altair, then having it work on the first try after laboriously entering it with toggle switches at MITS in Albuquerque, was a remarkable achievement.
What Allen did was write an 8080 emulator that ran on the -10. The 8080 is a simple CPU, so writing an emulator for it isn't hard.
https://pastraiser.com/cpu/i8080/i8080_opcodes.html
Then, their BASIC was debugged by running it on the emulator.
The genius was not the difficulty of doing that, it wasn't hard. The genius was the idea of writing an 8080 emulator. Wozniak, in comparison, wrote Apple code all by hand in assembler and then hand-assembled it to binary, a very tedious and error-prone method.
In the same time period, I worked at Aph, and we were developing code that ran on the 6800 and other microprocessors. We used full-fledged macro assemblers running on the PDP-11 to assemble the code into binary, and then download binary into an EPROM which was then inserted into the computer and run. Having a professional macro assembler and text editors on the -11 was an enormous productivity boost, with far fewer errors. (Dan O'Dowd wrote those assemblers.)
(I'm doing something similar with my efforts to write an AArch64 code generator. First I wrote a disassembler for it, testing it by generating AArch64 code via gcc, disassembling that with objdump and then comparing the results with my disassmbler. This helps enormously in verifying that the correct binary is being generated. Since there are thousands of instructions in the AArch64, this is a much scaled up version of the 8080.)
The Wozniak method was how I used to write 6502 assembler programs in high school since I didn’t have the money to buy a proper assembler. I wrote everything out longhand on graph paper in three columns. Addresses on the left, a space for the code in the middle and the assembler opcodes on the right, then I’d go through and fill in all the hex codes for what I’d written. When you work like that, it really focuses the mind because there’s not much margin for error and making a big change in logic requires a lot of manual effort.
2 replies →
Allen had to write the loader in machine code, which was toggled in on the Altair console. The BASIC interpreter itself was loaded from paper tape via the loader and a tape reader. The first BASIC program Allen ran on the Altair was apparently "2 + 2", which worked - i.e. it printed "4" I'd like to have such confidence in my own code, particularly the I/O, which must have been tricky to emulate on the Dec10.
> which must have been tricky to emulate on the Dec10
I don't see why it would be tricky. I don't know how Allen's 8080 emulator on the PDP-10 worked, but it seems straightforward to emulate 8080 I/O.
2 replies →
Fun facts, according to Jobs for some unknown reasons Wozniak refused to add floating point support to Apple Basic thus they had to license BASIC with floating point numbers from Microsoft [1].
[1] Bill & Steve (Jobs!) reminisce about floating point BASIC:
https://devblogs.microsoft.com/vbteam/bill-steve-jobs-remini...
Writing a floating point emulator (I've done it) is not too hard. First, write it in a high level language, and debug the algorithm. Then hand-assembling it is not hard.
What is hard is skipping the high level language step, and trying to do it in assembler in one step.
The thing is, Woz already wrote floating point routines which were included in the Apple II ROMs themselves that you could call with PEEK/POKE. They just were never integrated into the BASIC language itself!
http://retro.hansotten.nl/uploads/mag6502/Apples%20Hidden%20...
Also, though, how big was Apple Integer BASIC? As I understand it, you had an entire PDP-10 at your disposal when you wrote the Fortran version of Empire.
2 replies →
I've never understood floating point :-)
8 replies →
Floating point math was a key feature on these early machines, since it opened up the "glorified desk calculator" use case. This was one use for them (along with gaming and use as a remote terminal) that did not require convenient data storage, which would've been a real challenge before disk drives became a standard. And the float implementation included in BASIC was the most common back in the day. (There are even some subtle differences between it and the modern IEEE variety that we'd be familiar with today.)
I agree - it's a useful BASIC that can do math and fits in 4 or 8 kilobytes of memory.
And Bill Gates complaining about pirating $150 Altair BASIC inspired the creation of Tiny BASIC, as well as the coining of "copyleft".
I still have a cassette tape with Microsoft Basic for the Interact computer. It's got an 8080.
I remember my old Tandy Color Computer booting up and referencing Microsoft BASIC:
https://tinyurl.com/2jttvjzk
The computer came with some pretty good books with example BASIC programs to type in.
You should upload the audio to the Internet Archive!
I have a MS Extended Basic cassette for the Sol-20, also 8080 based.