The PHP Virtual Machine is a basic virtual machine written in PHP that accepts a set of instructions modeled loosely in the style of assembly, then executes them. In this first stage it has a 512 integer "RAM" and accepts 7 unique instructions. It also has to have the program coded directly into the script, as I haven't finished the input side of things yet.

The seven operations you can perform at this point are:

Using these basic language structures a simple countdown program can be implemented in 8 instructions.

0 SET(0,10)
1 SET(1,-1)
2 SET(2,4)
3 SET(3,10)
4 PRT(0,0)
5 ASC(3,0)
6 ADD(0,1)
7 BNZ(0,2)
8 STP(0,0)

Let's walk through this program step by step. To keep track of things, I'll refer to the memory as an array called RAM, like it is in the PHP VM implementation.

Our first instruction is to SET RAM[0] to the value 10.
After executing this the first five RAM values looks like this:
[10][0][0][0][0]

The next three are similar.
SET RAM[1] to -1
SET RAM[2] to 4
SET RAM[3] to 10
After execution RAM looks like this:
[10][-1][4][10][0]

Next I think that it is important to point out at this point what exactly we are holding each value in RAM for.

RAM[0] is our counter, to keep track of where we are in the countdown.

RAM[1] is our decrement variable, we will ADD it to RAM[0] to decrease it's value.

RAM[2] is the index of the instruction we want to jump back to on the instruction array. If you look at the program, you'll see that this corresponds to PRT(0,0).

Now, back to our program. We now hit PRT(0,0), so we print the integer value found at RAM[0], which is 10.

Next up is ASC(3,0), so we print the ASCII value of the integer found at RAM[3]. That is integer is 10, and the ASCII value of 10 is a newline character (\n).

After that is ADD(0,1). This requires us to add the value found in RAM[0] and RAM[1] together, then store it in RAM[0].

This is essentialy saying RAM[0] = 10 + (-1)

Our array of RAM looks like this afterwards:
[9][-1][4][10][0]

Now we reach the harder part, BNZ(0,2). What this is saying is "If the value found in RAM[0] is not equal to literal 0, then jump back to the instruction index that is equal to the value found in RAM[2]". Since, at this point RAM[0] is equall to 9, and not zero, we jump to the instruction at the value of RAM[2], which is 4. This is PRT(0,0).

Here we repeat instructions 4 through 7 another 8 times, with the RAM looking like this in each successive pass:
[8][-1][4][10][0]
[7][-1][4][10][0]
[8][-1][4][10][0]
[5][-1][4][10][0]
[4][-1][4][10][0]
[3][-1][4][10][0]
[2][-1][4][10][0]
[1][-1][4][10][0]
[0][-1][4][10][0]
At this point something new happens, instruction 8, BNZ(0,2) returns false, and the instruction pointer is allowed to increment naturally.

This gets us to STP(0,0), which simply ends the program.

I realize I'm not the best instructor, so you'll probably just want to get the source code and play around with it. You'll figure it out soon enough if you haven't got it already. Below is an example run of the program described in this document.

PHP VM Version 0.01 Source

PHP VM - Version 0.01
Loading Instructions
Padding RAM
Setting Environment
Executing

10
9
8
7
6
5
4
3
2
1

Finished - Running Time: 0.00021696 Seconds