6502 Assembly Language Lab (Old Version)

This page may be obsolete.
It contains historical information.

Purpose of this Lab
In this lab, you will learn some of the basics of 6502 assembly language, in preparation for learning more complex x86_64 and AArch64 assembly language.

Resources

• 6502 Wiki Page
• 6502 Emulator
• 6502 Emulator Example Code
• Opcode/Instruction References
  • 6502 Opcodes with Register Definitions
  • 6502 Opcodes with Detailed Operation Information

Lab 1

Setup

1. Enter the breakout room assigned to you. 2. Select one person to be the "Driver", who will type/operate the computer for the group. 3. The driver will open the 6502 Emulator at [1] (and, ideally, this lab page as well).

Sharing Results
Decide how group results will be shared between the members of the group. (Suggestion: consider using a git repository).

Save Your Work
The emulator does not save your work. Remember to periodically save it to a file (copy-and-paste the code).

Bitmap Code

3. Paste this code into the emulator:

	lda #$00	; set a pointer at $40 to point to $0200
	sta $40
	lda #$02
	sta $41

	lda #$07	; colour number

	ldy #$00	; set index to 0

loop:	sta ($40),y	; set pixel at the address (pointer)+Y

	iny		; increment index
	bne loop	; continue until done the page

	inc $41		; increment the page
	ldx $41		; get the current page number
	cpx #$06	; compare with 6
	bne loop	; continue until done all pages

4. Test the code by pressing the Assemble button, then the Run button. If the there are any errors assembling (compiling) the code, they will appear in the message area at the bottom of the page. Make sure the code is running correctly and that everyone in your group understands how it works. Try some experiments: use different colours, or draw only every second dot.

Writing Code

5. Write code to draw a green line across the top of the bitmap screen and a blue line across the bottom.

6. Extend the previous code to draw a yellow line down the left side of the screen and a purple line down the right side.

Performance

7. In the Resources section, the opcode/instruction references will tell you the number of machine cycles that each instruction will take. If your system is running at 1 MHz (a typical speed for a 6502 processor), each machine cycle will take 1 microsecond (uS). What is the total time that your code (from step 6) will take to execute?

Optional (Recommended): Experiments

Go back to the bitmap code above, and try these experiments:

1. Add this instruction after the loop: label and before the sta ($40),y instruction: tya
2. What visual effect does this cause, and how many colours are on the screen? Why?
3. Add this instruction after the tya: lsa
4. What visual effect does this cause, and how many colours are on the screen? Why?
5. Repeat the above tests with two, three, four, and five lsr instructions in a row. Describe and explain the effect in each case.
6. Repeat the tests using asl instructions instead of lsr instructions. Describe and explain the effect in each case.
7. Remove the tya and all asl and lsr instructions.
8. The original code includes one iny instruction. Test with one to five consecutive iny instructions. Describe and explain the effect in each case. Note: ensure that the Speed slider is on its lowest setting (left) for these experiments.

Write-Up

Post an entry on your blog describing your experiments in this lab. Include:

1. An introduction, so that someone who happens across your blog will understand the context of what you're writing about.
2. The results from the Writing Code portions of the lab, including the code, a description of how the code works, and the results produced.
3. Your answers to the Performance question, including a description of how you reached your answer.
4. Optional (Recommended): the results of the Experiments section (above), and your explaination for each observed result.
5. Your experiences with this lab -- your impressions of the Assembly Language, what you learned, and your reflections on the process.

Remember to follow the Blog Guidelines as you write.