About memory page, Branching and cycle

A page in 6502 is 256 bytes. It's pretty easy to understand:

$0000-00FF = page 0
$0100-01FF = page 1
$0200-02FF = page 1
...
$0F00-0FFF = page 15
$1000-11FF = page 16
...

You get the idea.

Regarding branch-related opcodes: the operand is a signed number. This is why branch opcodes have a limitation in range of -128 to +127 bytes from the PC. In this case, the branch is signed (MSB / bit 7 of the operand is set), indicating when the condition is met, the PC is to go backwards (e.g. branch backwards).

$FF - $FB = 4. Assuming the branch condition is met, the PC will have 4 subtracted from it, and will continue operation there. It's important to remember that the PC increments at the operand (the "$FB" portion), so when this instruction executes, the PC is actually at $C07D.

So, the disassembly in question therefore becomes this:



$C07D - 4 = $C079.

If you truly want to understand how it works: it's two's complement. I'd rather not explain why; here's a better description (see section "Relative Addressing").

Now take this example code:



In this case, the operand is positive (MSB / bit 7 of $0A is not set), which means the branch goes forward. $C07D + 10 = $C087. But wait, the branch there says $C088, so how did this happen? Because the PC is incremented by one after executing the instruction, so $C087 + 1 = $C088.

Branching itself is just a form of JMP, but is limited to the above range of bytes. Branching is conditional (you already know this), but it also saves a byte compared to JMP (JMP = 3 bytes, branch = 2 bytes).

Hope this helps.