Calculator Introduction

This page describes the calculator functions of the KIM Uno. Software archaeology is nice; 6502 programming is fun and interesting. But you have got to wonder what you are actually going to do, regularly, with something as obsolete as a KIM-1 in the 21st century...

The one useful role such technology still can perform well is to be a programmable calculator.

So the idea is to provide a 6502 programmable calculator, using the KIM-1 environment with minimal extensions. Basically, a feature to show floating point numbers with 7 digits and decimal point.

There's also normal non-programmable calculator mode (press SST for >1 second to toggle in and out of that mode). It simply changes the assignment of the keys on the KIM Uno keypad so you can enter and view floating point numbers interactively, and you should see this mode as much as a debugger for when you write calculator programs as it is a normal non-programmable calculator mode on its own.

if you think of early microcomputers as glorified calculators, it's surprising to find out just how hard it is to do simple floating point math on a CPU. Some of the very earliest programming on microprocessors - before operating systems even arrived - was thus to produce math routines. In 1976, MOS Technology published KIMATH, and Roy Rankin and Steve Wozniak published a competing math library in Dr. Dobb's Journal. In recent years, Charles R Bond has created a new 6502 math library, fltpt65, which is probably the last word in 6502 math programming. This is the one used in the KIM Uno. It's placed in a ROM starting at $5000.

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Quick Start: The KIM as 6502 Programmable Calculator

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There is a built-in demo program at $0210 that you can try. It asks you to enter two floating point numbers, and it then shows you the product of both. Next, it asks you for an integer number, used as a yes/no question. If you enter 00, the program ends by showing the last result again. If you enter 01, it shows the square root of the previous answer.

Enter 0210 [GO]              

Enter 4 [GO]                 

Enter 5 [GO]                 

View number, then press [GO]  (4*5=20.000)

Enter 01 [GO]                 (01 = yes, take square root)

View number, then press [GO]  (sqrt(20) = 4.472135)

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You can read the program either as hex code on the display, or use the disassembler or mini-assembler to list it on the serial port. Basically, simple calculator programs consist of

• JSR 70Ax calls to ask for floating point numbers;

• JSR 70xy calls to copy/swap floating point numbers between the 'registers' that store them for you;

• LDA XX statements to choose the math operation (00=add, 02=multiply, etc, see below);

• JSR 00 50 calls to execute calculations;

• JSR 70Dx calls to display floating point numbers;

• BEQ statements to do if-then statements.

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You're pretty much set if you know just the hex codes for three 6502 instructions used above, plus the addresses of the various JSR 'Ask/Display' calls. See further below for details.

Technical interlude: fltpt65

Fltpt65 requires you to:

• enter the two numbers you want to work with in its W1 and W2 registers,

• enter the operation you want it to perform in the 6502's A register,
  (using the KIM Monitor environment, you can put the value in $00F3, where the monitor will pick it up and store it in A for you just before starting program execution).

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Then, you call the program at $5000 (so 5000 [GO] on the KIM), and fltpt65 will provide the result in its W3 register. There also is a W4, which you can use as a calculator memory. The four registers are 8-byte locations in memory, starting at $0360.

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The above is simple enough. But W1 and W2 are encoded floating point numbers, not just single-byte integers, so they are hard to handle. Each W register is an array of 8 bytes that (ignore the following unless you care) represents a floating point number, BCD encoded, with 12 mantissa digits and 3 exponent digits supporting a maximum value of  +9.999999999 E+999.

The picture below shows the format of fltpt65 numbers. But the point is, this format is hard to use for humans. The KIM Uno has to provide some user interface to read and write W registers.