DAY 2. The Components Of A Z80 Program

novice intermediate advanced expert

Every Z80 program consists of several parts combined in a certain way. Today, we will take a look at these sections, using the program from yesterday as an example. For the sake of clarity, I have colorized common parts.

#DEFINE  ProgStart    $9D95
.ORG    ProgStart - 2
.DB     t2ByteTok, tAsmCmp
    LD     HL, 0
    LD     (PenCol), HL
    LD     HL, msg
    b_call(_PutS)        ; Display the text
    .DB "Hello world!", 0

Directives   MAROON

Directives are preceded by either a period or a "#". Their purpose is to give instructions to the assembler about how it is to interpret the source code. Directives are normally on the leftmost edge, but they can be indented any way you want. Here are some very candid descriptions of the functions of the directives used above.
These affect something called a listing file that TASM creates. Not really important, but have them in anyway.
#define text1 text2 Replaces all instances of text1 in the program — that aren't in quotation marks or part of larger words—with text2. Essentially just a Find/Replace command.
#include "filename" Inserts the contents of filename into the source code.
text .equ number Virtually identical to #define, when text2 is a number.
.db Specifies data.
.end Signifies the end of the source code. There are two here because TASM has this annoying tendency to ignore the last line in a file. the second one is redundant, but oh well.
.org number Specifies where in memory the program is loaded into (not exactly true), which is always $9D95. Notice the "- 2"? Don't worry about it now.

Comments   RED

Anything that follows a semicolon is a comment, and will be totally ignored by the assembler. Comments are used to explain the effect of, or reasoning behind sections of code. They may not seem very useful for all the noddy programs you will get to see, and that's probably true. But if you've been working on a project for over a month, and you're puzzling over what you were thinking when you wrote that instruction block, that is a divine sign that you should be commenting your code.

Instructions   PURPLE

These are the commands that tell the processor what to do. They are also all the processor knows how to do, so if you can't do something with instructions, you can't do it at all. There are over a hundred unique instructions, but fortunately the majority are just variations on a theme. Syntax note: instructions must be indented in the source.


Certain tasks, such as displaying text on the screen, are used so often that TI decided to make the code for them a permanent part of the calculator. This was a very good idea, since the actual coding for some of them is very long and complex (for example, our "Hello World!" program takes only a few dozen bytes of storage, but when the code for the ROM calls is added in, it adds up to several kilobytes). These miniprograms are stored in the calculator's ROM (hence the name). When the program encounters a ROM call, it transfers to the location in ROM where the code is stored, and returns to the program when finished. You execute a ROM call like this:
ROM calls must be tabbed in along with instructions.

Lots of people knock ROM calls because they believe them to be inherently slow and poorly implemented to boot. Okay, that's true, but no one can argue that for beginners, having simple to use prefabricated support software makes the difference between frustration and accomplishment.

Manifest Constants   GREEN

A much more in-depth discussion of manifest constants appears in a later chapter. Very briefly, a manifest constant is a symbolic representation of a number. Once a manifest constant has been defined, at every place in the source code it is encountered it is interpreted as the value it was defined as. There are three ways to define a manifest constant. You can use #define or .equ (see the table above), or you can use a label.
Labels are headings for sections of the program. They have to be on the leftmost edge, and end with a colon. You will learn what value a label represents a little later.

Throughout the course of this tutorial, when a new instruction or ROM call comes up, it's function will be fully (or at leat mostly) explained, along with any data you have to apply (called arguments or operands) to make it work. Kinda like these:
RET Quits the program and returns to the TI-OS. All programs must eventually execute this instruction.
_ClrLCDFull Clears the screen
For more information about these little gray boxes, check out the guide's style conventions.

Program Template

Programs always need some assembler commands in order to work right. When making your own programs, all you need to do is follow this format:
    #include    ""
.org    $9D93
.db    t2ByteTok, tAsmCmp
    Your program goes here.
The stuff in blue is required for every program. To save space, all example programs in this guide will omit it. If you want to test an example program, you will have to supply the required parts yourself. I would suggest you make a template file, then copy and rename it every time you want to test something.

This is part of Learn TI-83 Plus Assembly In 28 Days
Copyright (c) 2002, 2003, 2004 Sean McLaughlin
See the file gfdl.html for copying conditions