How does a branch instruction work on the 6502 with a relative offset of 0x00 (fetch / execute cycle)?

1
votes

Edit: i'm not using any debuggers since i don't intend on installing any software, but recommendations are appreciated

Suppose we stumble upon the instruction BPL $0x00

Base Address(PC): 0x400 value: BPL $00

Intuitively one might assume that the program will hang at address 0x400.

However, im more inclined to say, it will first fetch the opcode at 0x400, increment PC, decode that it needs to read an operand, fetch the operand, increment PC.

That leaves us at address 0x402. By then the processor would decode the instruction and opcode and execute before fetching the next opcode.

So what ends up happening then? does the program branch to address 0x400, or does it branch to address 0x402 like I predict?

2
6502
It just adds the value to the PC, so $402 as you predict. – Nick Westgate

2 Answers

5
votes

As commented already, the offset is added to the address following the instruction, so an offset of 00 makes the branch instruction add nothing if the branch is true. Be aware, however, that a NULL branch does have an effect on code execution, albeit a minor one involving timing, since a branch taken needs three cycles to complete, while a branch skipped needs only two.

LDA #45
BPL +00 ; branch taken, execution time 3 clock cycles
LDA #FE 
BPL +00 ; branch skipped, execution time 2 clock cycles

All of this information is provided in the 6500 Programming Manual, http://6502.org/documents/books/mcs6500_family_programming_manual.pdf, pp. 40-45.

3
votes

A branch instruction (if true) adds the given argument to the PC. In your program the command branches to 0x402 as you predicted.

With a simply simulator like on page http://skilldrick.github.io/easy6502/ you could test the behavior:

  LDA #$F1
label:
  BPL label
  LDA #$01
  BPL label2
label2:
  BRK

Disassembling results in:

Address  Hexdump   Dissassembly
-------------------------------
$0600    a9 f1     LDA #$f1
$0602    10 fe     BPL $0602
$0604    a9 01     LDA #$01
$0606    10 00     BPL $0608
$0608    00        BRK

So, as you see, the hexcode 0x10 (BPL) with argument 0x00 just goes to the next instruction (here to address at 0608).