Replica 1 Quick Reference, Apple 1 on the Raspberry Pi, and the return of IMSAI

Earlier I made a one page reference of useful addresses for the Replica 1. I've extended that into a more comprehensive four page quick reference. You can download it from here. That directory also has some documents I wrote like a summary of the 65C02 and 65816 processors and some errata for Apple 1 manuals.

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I recently got a Raspberry Pi board. This is a very small (credit card sized), low cost ($35) embedded desktop computer that can run Linux. For fun I compiled the POM1 Apple 1 Emulator on it. It ran quite well although it was a little slow on the Raspberry Pi's 600MHz ARM CPU. I've ordered a hardware expansion board for the Raspberry Pi called the Gertboard and plan to use if for some hardware experiments.

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I've started playing with the Propeller CPU on the Replica 1. It is quite an amazing chip - 8 CPUs with shared memory, a built-in high level language interpreter as well as machine language, and hardware I/O ports. It can even generate video (which it does for the Replica 1). I'll have more to say here as I explore it further.

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I'm looking forward to getting the Apple Cassette Interface when Briel computers starts shipping them. It sounds like big things are in store for Briel Computers. Vince Briel is planning to purchase the assets
of IMSAI computer. They were one of the first home computer manufacturers (along with Altair) and a portion of the company was still in business. He is hoping to restart a project they had to introduce a new replica of the original IMSAI 8080 computer.

Converting Between Floating Point and Human Readable Format

In my last post I mentioned 6502 code that can convert between ASCII and floating point formats that was written by Marvin L De Jong and published in the February and April 1981 issues of COMPUTE! magazine.

I've now entered it and ported it to the CC65 assembler to run on the Replica 1.

As often seems to be the case, there were a number of errors in the printed code, such as symbols that were never defined. The "#" sign was also not used to specify immediate addressing (this was apparently a coding convention that the author followed in all of his published code).

Fortunately it seemed that the generated machine code in the listing was correct, so in most cases I was able to use that as a reference to determine what the code should be.

I also had to combine the code in the first article with changes in the second one as there was no complete listing of the final software.

The code used some i/o routines for the AIM-65 machine so those needed to be be replaced with equivalents for the Replica 1.

After a couple of evenings of porting and testing I had the string to floating point and floating point to string functions working (De Jong refers to it as "BCD format" but it is really an ASCII string format). So, for example, I could now convert a string like "+6.02214E23" or "-1.2E-19" to floating point and back to ASCII. I then updated my little test/demo program to support calling the new routines.

The floating point format used by the De Jong code is a little different from the format used by the Woz code, but I was able to convert them easily enough.

The final result is the demo program which exercises all the functions. Here is an example run:

FLOATING POINT DEMONSTRATION PROGRAM

F - FIXED TO FLOATING POINT
P - FLOATING TO FIXED POINT
L - NATURAL LOG
N - COMMON LOG
E - EXPONENTIAL
A - FLOATING POINT ADD
S - FLOATING POINT SUBTRACT
M - FLOATING POINT MULTIPLY
D - FLOATING POINT DIVIDE
B - STRING TO FLOATING POINT
T - FLOATING POINT TO STRING
? - THIS HELP SCREEN
X - EXIT

SELECT A FUNCTION: B
STRING TO FLOATING POINT
ENTER FP STRING: 1234
FLOATING POINT IS: 8A 4D2000

SELECT A FUNCTION: T
FLOATING POINT TO STRING
ENTER EXPONENT AND MANTISSA: 8A 4D2000
FLOATING POINT IS: 1234

SELECT A FUNCTION: B
STRING TO FLOATING POINT
ENTER FP STRING: 6.02214E23
FLOATING POINT IS: CE 7F8616

SELECT A FUNCTION: T
FLOATING POINT TO STRING
ENTER EXPONENT AND MANTISSA: CE 7F8616
FLOATING POINT IS: 60221399.E16

With the facilities here you could implement a math program like a scientific calculator. I may do that at some point.

I learned a little about floating point code with this project. It also reminded me that back at the time this code was written there was no standard for representing floating point numbers. That was addressed by the IEEE Standard for Floating-Point Arithmetic (IEEE 754) and most modern CPU chips that have floating point hardware now support this standard, and language compilers typically support it in their run-time libraries. CPUs that lack floating point hardware typically emulate it in software, using code much like the 6502 code here, but more complex and typically written in C.

As usual, all of the code can be found here and you are welcome to use it.

6502 Floating Point Routines

In the August 1976 issue of Dr. Dobb's Journal Steve Wozniak and Roy Rankin published a listing of floating point math routines for the 6502. The code is portable and provides routines for floating point add, subtract, multiply and divide as well as natural and common log and exponential functions and conversion between fixed and floating point. A version of this code was later included in the firmware for the Apple II.

The article in PDF and text format is available from 6502.org  so I thought it would be fun to get it running on the Replica 1. I took the code and made a few small changes to get it to assemble under the CC65 assembler. I also included some later fixes that were published in Dr. Dobb's.

I then wrote an interactive program that allows the different functions to be executed. I did some minimal testing and all the routines seemed to be working quite well.

Here is a sample run of the program (input by the user is in bold):

FLOATING POINT DEMONSTRATION PROGRAM


F - FIXED TO FLOATING POINT
P - FLOATING TO FIXED POINT
L - NATURAL LOG
N - COMMON LOG
E - EXPONENTIAL
A - FLOATING POINT ADD
S - FLOATING POINT SUBTRACT
M - FLOATING POINT MULTIPLY
D - FLOATING POINT DIVIDE
? - THIS HELP SCREEN
X - EXIT


SELECT A FUNCTION: F
FIXED TO FLOATING POINT
ENTER 16-BIT HEX NUMBER: 0123
FLOATING POINT IS: 88 48C000


SELECT A FUNCTION: P
FLOATING TO FIXED POINT
ENTER EXPONENT AND MANTISSA: 88 48C000
FIXED POINT IS: 0123


SELECT A FUNCTION: F
FIXED TO FLOATING POINT
ENTER 16-BIT HEX NUMBER: 0456
FLOATING POINT IS: 8A 456000


SELECT A FUNCTION: M
FLOATING POINT MULTIPLY
ENTER EXPONENT AND MANTISSA: 88 48C000
ENTER EXPONENT AND MANTISSA: 8A 456000
RESULT IS: 92 4EDC20


SELECT A FUNCTION: P
FLOATING TO FIXED POINT
ENTER EXPONENT AND MANTISSA: 92 4EDC20
ERROR OCCURRED AT ADDRESS $1FE6


SELECT A FUNCTION: A
FLOATING POINT ADD
ENTER EXPONENT AND MANTISSA: 88 48C000
ENTER EXPONENT AND MANTISSA: 8A 456000
RESULT IS: 8A 579000


SELECT A FUNCTION: P
FLOATING TO FIXED POINT
ENTER EXPONENT AND MANTISSA: 8A 579000
FIXED POINT IS: 0579


SELECT A FUNCTION: X

This code could be used to write assembly language programs that need to do floating point math. One piece is missing though -- conversion between the floating point representation and one that can be input by a user or printed. This is usually done using Binary Coded Decimal (BCD).

A little research found a couple of articles by Marvin L De Jong in the February and April 1981 issues of COMPUTE! magazine that described BCD to floating point and floating point to BCD conversion routines. While the floating point format he used was slightly different from what was used in Woz's code, I'm hoping that I can adapt it. So that is my next little project.

All of the code can be found here on github.

Musing on Kit Building

I love building electronic kits, and there is a long tradition of kit building among people who tinker with electronics, including amateur radio operators, that goes back almost 100 years.

That tradition continues today with products like the replica computers offered by Briel Computers and amateur radio kits from companies like Elecraft.

Despite predictions that surface mount technology and the reduced emphasis on electricity and electronics in education would mean the demise of kits, the popularity of kit building has grown in the last ten years or so, as part of what is now often called the "maker" movement.

While kits have differing levels of complexity in terms of the number of parts involved and effort to assemble them, it seems to me that there are four fundamentally different levels of kit building.

Let me expound my theory, which I think is original if not particularly insightful.

Level 1: Assembling From Pre-built Components or Modules.

Kits at this level involve assembling pre-built components and mechanically connecting pieces together but doesn't involve any soldering, or if it does, just simple soldering of wires and connectors.

I've heard people talk about "building their own computer" and more often than not they mean building one at this level -- taking a commercial motherboard, case, and power supply, and connecting them together into a working computer.

Examples of this category include the pre-assembled Apple Replica 1, the Arduino embedded controller or the Elecraft K3 amateur radio transceiver.

An Arduino Embedded Controller

This can be a good way to get started in electronics and build up confidence before you are ready to pick up a soldering iron.

Level 2: Building a Commercial Kit 

The next level involves assembly of a kit that requires soldering as well as mechanical assembly. You need to be able to solder competently and identify electronic components. You don't need to understand the circuit being built and don't need expensive test equipment although a Digital Multi Meter (DMM) is very useful.

The kit may include a professional case, power supply, etc. or may leave it up to the user to provide these.

The Heathkit company was the premiere source of kits of this type for many years and was renowned for the quality of its manuals and the professional look of the kits when assembled.

Briel Computers and many other companies offer similar kits of this type.

An Assortment of Heathkit Equipment

Level 3: Building From a Provided Design

The next step up is to take an electronic design, such as one described in a magazine or book article, or possibly as little as a schematic diagram, and realizing it as working device.

This involves finding and sourcing all the components, figuring out how to lay out the parts and deciding what construction technique to use (such as simple point to point wiring, a printed circuit board, or others such as "ugly" construction style).

This level offers more risks and challenges. There may be hidden gotchas in the design that will depend on the parts used or how it is assembled. There make even be errors in the design.

The builder needs to understand the circuit although not how all component values were determined. You may need more extensive test equipment such as an oscilloscope, signal generators, etc. in order to test and debug it. You may need tune or calibrate the circuit for proper operation.

This was how a lot of equipment was built by radio amateurs in the old days and still today. Circuits were often published in books like the Radio Amateur's Handbook or periodicals like QST magazine.

It is very rewarding as the end result is unique and there are often opportunities for making modifications.

An example from amateur radio is a transceiver (transmitter and receiver) called the the Ugly Weekender that was published in the amateur radio magazine QST in the 1980s. I built this unit using "ugly" construction on a bare copper board and built it into a case.

My Version of The Ugly Weekender

Level 4: Building Your Own Design

This is the ultimate level where you are no longer building a kit, but your own design.

You design the circuit yourself (possibly based on some existing design) and assemble it. To be successful you need to fully understand the circuit. You may even want to simulate the design using circuit simulation tools, or at least prototype key portions of it to verify the design.

This level of project can take months or more depending on the complexity and the time available to spend on it. It is the most risky but also the most rewarding if successful. If you succeed at this you've reached the "black belt" level.

A small example could be the RAM/EEPROM card I designed and built for the Apple Replica 1.

Replica 1 RAM/EEPROM Card

In summary, kit building can be categorized into the four levels I outlined. In practice there is often a grey area between the levels.

So the next time you work on a kit or electronic project, maybe you will ponder at what level you are and maybe consider moving your kit building prowess up to the next level.