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Atari 2600 Programming for Newbies

Session 19: Addressing Modes

By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Page Table of Contents

Original Session

Are we having fun, yet?

 

We're already familiar with a few ways of loading numbers into the 6502's registers, and storing numbers from those registers into RAM or TIA registers. We'll re-visit those methods we know about, learn some new ones (not all of the 6502's addressing modes, but enough to get by with).

 

This session we're going to have a bit of a look at the various ways that the 6502 can address memory, and how to write these in source code.

 

 

 

 

A, X, and Y Registers

As you should be aware by now, the 6502 has three registersA, X, and Y. 'A' is our workhorse register, and we use this to do most of our loading, storing, and calculations. X and Y are index registers, and we generally use these for looping, and counting operations. They also allow us to access 'lists' or tables of data in memory.

 

Let's start with the basics. To load and store actual values to and from registers, we can use the following…


    lda #$80    ; load accumulator with the number $80
                ; (=128 decimal)

    lda $80     ; load accumulator with contents of
                ; memory location $80



    sta #$80    ; meaningless!  DASM will kick a fit.
                ; You can't store to a number!

    sta $80     ; store accumulator's contents to
                ; memory location $80





    ldx #$80    ; load x-register with the number $80



   ; etc..

All registers can load numbers directly (called 'immediate values'). The above examples show the accumulator being loaded with #$80 (the number 128) and also the X register being loaded with the same value. You can do this with the Y register, too.

 

You can't STORE the accumulator to an immediate value. This is a meaningless concept. It's like me asking you to put a letter in your three. You may have a post-box numbered 'three', but you don't have a 'three'.

 

 

 

 

Absolute Addressing

All registers can load and store values to memory addresses by specifying the location of that address (or, of course, a label which equates to the location of that address). For example, the following two sections of code are equivalent…


    lda $F000  ; load accumulator with contents of $F000




    ; or. . .



where = $F000





    lda where   ; ditto (correction made 7/7/2003)

As noted, the above will work for X and Y registers, too. This form of addressing (addressing means "how we access memory") is called 'absolute addressing'. Earlier we covered how the 6502 addresses code over a 16-bit memory range (that is, there are 2^16 distinct addresses that the 6502 can access, ranging from 0 to $FFFF). To form a 16-bit address, the 6502 uses pairs of bytesand these are always stored in little-endian format (which means that we put the low-byte first, and the high-byte last). Thus, the address $F023 would be stored in memory as two bytes in this order … $23, $F0.

 

Now, when DASM is assembling our code, it converts the mnemonic we write for an instruction (eg: 'lda') into an opcode (a number) which is the 6502's way of understanding what each instruction is meant to do. We already encountered the mnemonic 'nop' which converted into $EA. Whenever the 6502 encountered an $EA as an instruction, it performed a 2-cycle delay (it 'executed' the NOP).

 

We've briefly covered how each 6502 instruction may have one or two additional parametersthat is, there's always an opcodebut there may be one or two additional bytes following the opcode. These bytes hold things such as address data, or numeric data. For example, when we write 'lda #$56', DASM will place the bytes $A9, $56 into the binary. The 6502 retrieves the $A9, recognizes this as a 'lda' instruction, then fetches the next byte $56 and transfers this value into the accumulator.

 

To signify absolute addresses, the two bytes of the address are placed in little-endian format following the opcode. If we write 'ldy $F023'indicating we wish to load the contents of memory location $F023 into the Y register, then DASM will put the bytes $AC, $23, $F0 into our binary. And the 6502 when executing will retrieve the $AC, recognize it as a 'ldy' instruction which requires a two-byte addressand then fetches the address from the next two bytes, giving $F023and THEN retrieving the contents of that memory location and transferring it into the y register.

 

 

 

 

Zero-Page Addressing

As you can see, this division of 16-bit addresses into low and high byte pairs essentially divides the memory map into 256 'pages' of 256 bytes each. The very first page (with the high-byte equal to 0) is known as 'zero-page', and this is treated a bit differently to the rest of memory. To optimize the space required for our binary, the 6502 designers decided that they would include a special version of memory addressing where, if the access was to zero page (and thus the high byte of the memory address is 0), then you could use a different opcode for the instruction and only include the low-byte of the address in the binary. This form of addressing is known as zero-page addressing.

 

As with our above example, if we were accessing memory location $80 (which is the same as $0080remember, leading zeroes are superfluous when writing numbers), then we *COULD* have an absolute access to this location (with the bytes $AC, $80, $00interpreted in a similar fashion as described above). But DASM is smartand it knows that when we are accessing zero-page addresses, it uses the more efficient (both smaller code-size and faster execution) form of the instruction, and instead places the following in our binary … $A4, $80. The 6502 recognizes the opcode $A4 as a 'ldy' instruction (as was the $AC) but in this case only one byte is retrieved to form the low byte of the address, and the high byte is assumed to be 0.

 

Mostly we can rely on DASM to choose the best form of addressing for us.

 

So far, we have seen that what we can do with all the registers is essentially the same. Unfortunately, this is not the case with all the addressing modes! The 6502 is not 'orthogonal'The registers aren't all equal. Some operations require certain registers. For example, you can only do math with the A register and if you want to set the stack pointer, you have to use the X register. There are many examples.
(Adapted from a post by Ed Fries)
and this has some bearing on our choice of which register to use for which purpose, when designing our kernel.

 

OK, so now we should know what is meant by 'absolute addresses' and 'zero page addresses'. Pretty simple, really. Both refer to the address of memory that the 6502 can theoretically accessand zero page addresses are those in the range $0000 to $00FF inclusive.

 

 

 

 

Absolute,X, Absolute,Y, Zero Page,X, and Zero Page,Y

The session discussing Initialization introduced an efficient way of clearing memory in a loop, using a register to run through 256 bytes, and storing 0 to the memory location formed by adding the contents of the x register to a fixed memory address. These addressing modes (using the X or Y register to add to a fixed memory address, giving a final address for access) are known as 'Absolute,X' and 'Absolute,Y' and 'Zero Page,X' and 'Zero Page,Y'. It is probably a good idea now to track down a good 6502 book


    ldx #1

    lda $23,x    ; load accumulator with contents of 
                 ; location 36 (=$24)

    ldy $23,x    ; load Y register with contents of
                 ; location %100100



    ldy #2

    ldx $23,y    ; load X register with contents of
                 ; location $25

    lda $23,y    ; load accumulator with contents of
                 ; location $25

That last line is interestingan example of the non-orthogonality of our instruction set. All of the above examples deal with zero-page addresses (that is, the high byte of the address is 0). Theoretically, these instructions don't need to include the high-byte in the address parameters in the binary. However, there is no 'zero page,y' load for the accumulator! There is a zero page,x one, though. Its a bit bizarre :)

 

So DASM will assemble 'ldx $23,y' to a zero page,y instruction2 bytes longbut it will assemble 'lda $23,y' to an absolute,y instruction3 bytes long. Such is life.

 

These zero page indexed instructions have a catchthe final address is always always always a zero page address. So in the following example…


    ldy #1

    lda $FF,y

Since (as we just discussed) this is an absolute indexed instruction, the accumulator is loaded with the contents of memory location $100. However, the following…


    ldy #1

    ldx $FF,y

Since this will assemble to a zero page indexed instruction, the final address is always zero-page (the high byte is set to 0 after the index register is added)so we will actually be accessing the contents of memory location 0 (!!). That is, the address is formed by adding the y register and the address ($FF+1 = $100) and dropping the high-byte. Something to be very aware of!

 

 

 

 

Absolute Indexed Addressing Modes

Absolute indexed addressing modes are handy for loading values from data tables in ROM. They allow us to use an index register to step (for example) the line number in a kernel, and use the same register to access playfield values from tables. Consider this (mockup) code…

 

      ldx #0   ; line #

Display

      lda MyPF0,x     ; load a value from the data table "MyPF0"

      sta PF0

      lda MyPF1,x     ; use table "MyPF1"

      sta PF1

      lda MyPF2,x     ; use table "MyPF2"

      sta PF2



      sta WSYNC

      inx

      cpx #192

      bne Display



     ; other stuff here



      jmp StartOfFrame





MyPF0

      .byte 1,2,3,4,5,6  ;...etc 192 bytes of data here, giving data
                         ; for PF0

MyPF1

      .byte 234,24,1,23,41,2 ; PF 1 data (should be 192 bytes long)

MyPF2

      .byte 64,244,31,73,43,2,0,0 ; PF 2 data (should be 192 bytes long)

 

The above code fragment uses tables of data in our ROM. These tables contain the values which should be written to the playfield registers for each scanline. The x register increments once for each scanline, and our absolute,x load for each playfield register will load consecutive values from the appropriate tables.

 

Then, creating pretty graphics becomes simply a matter of putting the right values into those tables MyPF0, MyPF1, and MyPF2. This is where building tools to convert from images to data tables becomes extremely useful! We'll cover more of this way of doing things when we complete our sessions on asymmetrical playfields. The plan is to use a tool to create these data tables, and simplify our kernel by using data tables to display just about any asymmetrical image we want!

 

 

 

 

 

Summary

Soon we'll cover the remaining 6502 addressing modes, and also discuss the 6502's stack.

 

 

 

 

 

Exercises

  1. Use this method of absolute,x table access to modify or create a kernel which loads the graphics data from tables. Separate each playfield register into its own table, as above.
  2. Can you extend this system to asymmetrical playfield? Don't worry, we're going to give a complete asymmetrical playfield kernel (and tools!) in the next session.
  3. How would you incorporate color changes into this system (ie: if you wanted clouds on the left, sun on the right)?
  4. Each table requires 1 byte of ROM per PF register per scanline. Can you think of ways to reduce this requirement? What trade-offs are necessary when reducing the table size?
  5. Find a 6502 cycle-timing reference, and try to calculate exactly how many cycles each instruction in your kernel is taking. Add-up all the instructions on each line, and work out just how much time you have left to do "all the other stuff". Such as sprite drawing!

 

 

 

Other Assembly Language Tutorials

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.

 

Amazon Stuff

 

< Previous Session

 

 

Next Session >

 

 

 

 

Session Links

Session 1: Start Here

Session 2: Television Display Basics

Sessions 3 & 6: The TIA and the 6502

Session 4: The TIA

Session 5: Memory Architecture

Session 7: The TV and our Kernel

Session 8: Our First Kernel

Session 9: 6502 and DASM - Assembling the Basics

Session 10: Orgasm

Session 11: Colorful Colors

Session 12: Initialization

Session 13: Playfield Basics

Session 14: Playfield Weirdness

Session 15: Playfield Continued

Session 16: Letting the Assembler do the Work

Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)

Session 19: Addressing Modes

Session 20: Asymmetrical Playfields (Part 3)

Session 21: Sprites

Session 22: Sprites, Horizontal Positioning (Part 1)

Session 22: Sprites, Horizontal Positioning (Part 2)

Session 23: Moving Sprites Vertically

Session 24: Some Nice Code

Session 25: Advanced Timeslicing

 

 

 

 

Useful Links

Easy 6502 by Nick Morgan

How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.

 

 

Atari Roots by Mark Andrews (Online Book)

This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).

 

 

Machine Language For Beginners by Richard Mansfield (Online Book)

This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.

The Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

Nice, simple instruction set in little boxes (not made out of ticky-tacky).

 

 

The Second Book Of Machine Language by Richard Mansfield (Online Book)

This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.

6502 Instruction Set

An easy-to-read page from The Second Book Of Machine Language.

 

 

6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

 

 

6502.org

Continually strives to remain the largest and most complete source for 6502-related information in the world.

NMOS 6502 Opcodes

By John Pickens. Updated by Bruce Clark.

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

Registers

Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.

 

 

Stella Programmer's Guide

HTMLified version.

 

 

Nick Bensema's Guide to Cycle Counting on the Atari 2600

Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.

 

 

How to Draw A Playfield by Nick Bensema

Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.

 

 

Cart Sizes and Bankswitching Methods by Kevin Horton

The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.

 

 

Atari 2600 Specifications

Atari 2600 programming specs (HTML version).

 

 

Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.

 

 

MiniDig

Atari 2600 programming site based on Garon's "The Dig," which is now dead.

 

 

TIA Color Charts and Tools

Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.

 

 

The Atari 2600 Music and Sound Page

Adapted information and charts related to Atari 2600 music and sound.

 

 

Game Standards and Procedures

A guide and a check list for finished carts.

 

 

Stella

A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.

 

 

JAVATARI

A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.

 

 

batari Basic Commands

If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.

 

 

Back to Top

 

 

Affirmations

I'm a money magnet. Good things happen to me. I get things done. I'm happy. I'm healthy. I'm smart. I'm creative. I'm a nice person. I'm successful. I'm good with money. I'm honest. I'm trustworthy. I'm responsible. I'm wise. I'm easygoing. I'm clear-minded. I'm sober. I'm calm. I'm thankful. I'm satisfied. I'm forgiving. I'm confident. I'm kind. I'm considerate. I'm likeable. I'm friendly. I'm loving. I'm loveable. I'm joyful. I'm playful. I'm full of energy. I'm fun to be around. I'm a good friend. I'm eternal. I'm powerful. I'm a being of light. I'm a spirit wearing a body.

 

In Case You Didn't Know

 

Trump's Jab = Bad

Did you know that Trump's rushed Operation Warp Speed rona jab has less than one percent overall benefit? Some people call it the depopulation jab and it has many possible horrible side effects (depending on the lot number, concentration, and if it was kept cold). Remember when many Democrats were against Trump's Operation Warp Speed depopulation jab, then they quickly changed their minds when Biden flip-flopped and started pushing it?

 

Some brainwashed rona jab cultists claim that there are no victims of the jab, but person after person will post what the jab did to them, a friend, or a family member on web sites such as Facebook and they'll be lucky if they don't get banned soon after. Posting the truth is “misinformation” don't you know. Awakened sheep might turn into lions, so powerful people will do just about anything to keep the sheep from waking up.

 

Check out these videos:

If You Got the COVID Shot and Aren't Injured, This May Be Why

Thought Experiment: What Happens After the Jab?

The Truth About Polio and Vaccines

What Is Causing the Mysterious Self-Assembling Non-Organic Clots and Sudden Deaths?

Full Video of Tennessee House of Representatives Health Subcommittee Hearing Room 2 (The Doctors Start Talking at 33:28)

 

 

H Word and I Word = Good

Take a look at my page about the famous demonized medicines called The H Word and Beyond. You might also want to look at my page called Zinc and Quercetin. My sister and I have been taking zinc and quercetin since the summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe). Here's one more page to check out: My Sister's Experiences With COVID-19.

 

 

B Vitamins = Good

Some people appear to have a mental illness because they have a vitamin B deficiency. For example, the wife of a guy I used to chat with online had severe mood swings which seemed to be caused by food allergies or intolerances. She would became irrational, obnoxious, throw tantrums, and generally act like she had a mental illness. The horrid behavior stopped after she started taking a vitamin B complex. I've been taking Jarrow B-Right (#ad) for many years. It makes me much easier to live with. I wonder how many people with schizophrenia and other mental mental illnesses could be helped by taking a B complex once or twice a day with meals (depending on their weight)?

 

 

Soy = Bad

Unfermented soy is bad!When she stopped eating soy, the mental problems went away.” Fermented soy doesn't bother me, but the various versions of unfermented soy (soy flour, soybean oil, and so on) that are used in all kinds of products these days causes a negative mental health reaction in me that a vitamin B complex can't tame. The sinister encroachment of soy has made the careful reading of ingredients a necessity.

 

I wouldn't be surprised to find out that unfermented soy is the main reason why so many soy-sucking Democrats in the USA seem to be constantly angry and have a tendency to be violent when hearing words or reading signs that they don't agree with. If I unknowingly eat something with unfermented soy in it, I get irritable, angry, and feel like breaking things, so it's not the placebo effect. Scientists in the future will probably find out that unfermented soy can make people angry. We already know that food sensitivities cause mood changes. It took me over a decade to figure out that unfermented soy was affecting my mood. What if millions of people are having a similar reaction to soy and don't even know it? Some people eat it and drink it every day.

 

I started taking AyaLife (99% Pure CBD oil) as needed in April of 2020. So far it's the only thing that helps my mood when I've mistakenly eaten something that contains soy. AyaLife is THC-free (non-psychoactive) and is made in the USA. I also put a couple dropper fulls under my tongue before leaving the house or if I just need to calm down.

 

It's supposedly common knowledge that constantly angry Antifa-types basically live on soy products. What would happen if they stopped eating and drinking soy sludge and also took a B complex every day? Would a significant number of them become less angry? Would AyaLife CBD oil also help?

 

 

Wheat = Bad

If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by Ken D Berry, William Davis, and Ivor Cummins. It seems that most people should avoid wheat, not just those who have a wheat allergy or celiac disease. Check out these books: Undoctored (#ad), Wheat Belly (#ad), and Eat Rich, Live Long (#ad).

 

 

Negative Ions = Good

Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.

 

 

Litterbugs = Bad

Never litter. Toss it in the trash or take it home. Do not throw it on the ground. Also remember that good people clean up after themselves at home, out in public, at a campsite and so on. Leave it better than you found it.

 

 

Climate Change Cash Grab = Bad

Seems like more people than ever finally care about water, land, and air pollution, but the climate change cash grab scam is designed to put more of your money into the bank accounts of greedy politicians. Those power-hungry schemers try to trick us with bad data and lies about overpopulation while pretending to be caring do-gooders. Trying to eliminate pollution is a good thing, but the carbon footprint of the average law-abiding human right now is actually making the planet greener instead of killing it.

 

Eliminating farms and ranches, eating bugs, getting locked down in 15-minute cities, owning nothing, using digital currency (with expiration dates) that is tied to your social credit score, and paying higher taxes will not make things better and “save the Earth.” All that stuff is part of an agenda that has nothing to do with making the world a better place for the average person. It's all about control, depopulation, and making things better for the ultra-rich. They just want enough peasants left alive to keep things running smoothly.

 

Watch these two videos for more information:

CO2 is Greening The Earth

The Climate Agenda

 

 

How to Wake Up Normies

Charlie Robinson had some good advice about waking up normies (see the link to the video below). He said instead of verbally unloading or being nasty or acting like a bully, ask the person a question. Being nice and asking a question will help the person actually think about the subject.

 

Interesting videos:

Charlie Robinson Talks About the Best Way to Wake Up Normies

Georgia Guidestones Explained

The Men Who Own Everything

Disclaimer

View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.

 

Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.

 

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