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Register Multiplication Problem

This problem is part of the larger CPU Register Math project. It will simulate how multiplication can be performed with the 8-bit register 6502 CPU.

For this problem, you are to cannibalize any available code and write a pair of scripts which will accept decimal inputs for the multiplicand and multipier from the user and show how registers in the CPU can be used to find the product of their binary equivalents.

Your code should produce representations of the CPU registers which look like the one below.

Add Register X to Register A

Provide a short description of the action being taken.

To indicate which register has just been manipulated, display the row entries in red.

If a register has not as yet been used, have a query (?) represent each bit. (In reality, each bit in the CPU would contain a 1 or 0.)

Your solution should utilize two scripts.

1. The data entry script: multiply.html
   1. Explain the purpose of the scripts
   2. Accept the multiplicand and multiplier as inputs in a form
   3. Using a "Multiply!" button, send the data to a PHP script called multiply.php
2. The simulation page: multiply.php
   It should have four steps, the last of which is executed repeatedly and contains two steps.
   1. Zero out the accumulator
   2. Load the multiplicand into the X register
   3. Load the multiplier into the Y register
   4. Until the value in the Y register is zero,
      1. Add the value in the X register to the one in the accumulator, putting the result in the accumulator
      2. Decrement the value in the Y register

To see how this could work, view this possible solution.

Notes

• Given that you will print register information numerous times, it would be wise to create a printregister() function (which could also be used in the other related problems).

  A default color can be specified in the function declaration by doing something such as the following:
  function printregister($label, $bin, $color="BLACK")
  This allows you to call the function with either two or three arguments. If you use two arguments, $color will have the value "black"; if you use three, the value passed to the function will be used.

• Since there are two instances of register addition, it would be useful to write a function for adding two binary values, bit by bit.