Introducing the Instruction Set Part 3

This segment of the tutorial introduces branches, particularly conditional branches and function calls. This is Part 4 of a series. If you haven't yes, you may wish to review at least Part 1 and Part 2, and possibly Part 3 as well.

Unconditional Branches and Jumps

Unconditional branches are branches that are always taken. Jump instructions do essentially the same thing. The following table lists the instructions:

Mnemonic	Description	Cycles	Size
B	Branch to label	9	2 words
J	Jump to label	12	3 words
JD	Jump to label while disabling interrupts	12	3 words
JE	Jump to label while enabling interrupts	12	3 words

As you can see, the primary difference between branches and jumps is that branches are smaller and faster. Branches encode their "target address," the address being jumped to, as a relative displacement to the current address. Jumps, on the other hand, store the actual address of the target. In most cases, especially in a 16-bit ROM, there are few reasons to use a J instruction, although the combination instructions, JD and JE can be useful.

There is also a pseudo-instruction, JR, that allows "jumping to a location held in a register." It is really a pseudonym for "MOVR Rx, R7". Because it is a MOVR instruction, it will modify the Sign Flag and Zero Flag, which may be confusing if you're not expecting it.

Indirect Branches and Jump Tables

"It was a 'Jump to Conclusions' mat. You see, it would be this mat that you would put on the floor... and would have different conclusions written on it that you could jump to." -- Tom Smykowski, Office Space

Wrapping Up

At this point, you may wish to view the parts of this tutorial:

• Introducing the Instruction Set Part 1: The CPU, Memory and Registers; Primary Instructions and Addressing Modes
• Introducing the Instruction Set Part 2: Single Register and Implied Operand Instructions
• Introducing the Instruction Set Part 3: Shift and Rotate Instructions

Or, you can return to the Programming Tutorials index.