Back in the dark ages (the 1980's), people like myself coded on Apple][ computers. If you were good you coded in Applesoft BASIC or Integer BASIC. If you were really geeky you coded in Assembly language on the 6502 processor. The Apple][ OS was coded in assembly language, so if you really wanted to understand what was going on inside your computer, you needed to learn assembly language and you needed to learn about the 6502.
Obviously, the first thing you would do was write your own code to read and write disks. It was a big deal in those days to be able to copy 5.25" floppies that had games on them. For the serious geeks, it was much less interesting to play the games, and much more interesting to figure out how to copy them. One technique game manufacturers used was to write data between the tracks, by moving the drive arm 1/2 tracks and 1/4 tracks. Another technique was to modify the sector header bytes from the usual $D5 $AA $96 to something else. This change disabled standard Apple DOS from finding the sectors, and therefore made them unreadable to anyone other than the manufacturer of the game.
In order to be able to inspect disks, read sectors, read between tracks and so on, I wrote a small program that would enable me to inspect disks easily. I was young, so I simply called the program "M". You can see the source here. It's most likely that I used the LISA assembler, judging by the syntax of the source code. "M" allowed me to put a disk in the floppy drive and then using keyboard commands navigate through the disk and look at the contents, at a byte level.
There are lots of other 6502 assembler programs around the internet, including a nice archive at 6502.org. I had a lazy Saturday afternoon, so I decided to write an ANTLR4 grammar for LISA assembler. You can see it here. This grammar produces Java or C# (if you have the C# ANTLR Target) code which can parse LISA assembler. It's the first step to writing a Java or C# assembler for 6502 assembly code.
ANTLR has a useful feature where it can parse input and produce LISP-like output showing the fully parsed program. This feature is primary useful for debugging; it's an easy way to look at the AST in text format. Here is the LISP-like output from running my ANTLR grammar on "M". Of course, the parser and lexer that ANTLR produces in Java or C# does not output this LISP-like string, it produces an AST. A proper assembler would then walk that AST and output binary opcodes.
Here is an example of Bubble sort, coded in assembler. This was cut-pasted from 6502.org.
;THIS SUBROUTINE ARRANGES THE 8-BIT ELEMENTS OF A LIST IN ASCENDING ;ORDER. THE STARTING ADDRESS OF THE LIST IS IN LOCATIONS $30 AND ;$31. THE LENGTH OF THE LIST IS IN THE FIRST BYTE OF THE LIST. LOCATION ;$32 IS USED TO HOLD AN EXCHANGE FLAG. SORT8 LDY #$00 ;TURN EXCHANGE FLAG OFF (= 0) STY $32 LDA ($30),Y ;FETCH ELEMENT COUNT TAX ; AND PUT IT INTO X INY ;POINT TO FIRST ELEMENT IN LIST DEX ;DECREMENT ELEMENT COUNT NXTEL LDA ($30),Y ;FETCH ELEMENT INY CMP ($30),Y ;IS IT LARGER THAN THE NEXT ELEMENT? BCC CHKEND BEQ CHKEND ;YES. EXCHANGE ELEMENTS IN MEMORY PHA ; BY SAVING LOW BYTE ON STACK. LDA ($30),Y ; THEN GET HIGH BYTE AND DEY ; STORE IT AT LOW ADDRESS STA ($30),Y PLA ;PULL LOW BYTE FROM STACK INY ; AND STORE IT AT HIGH ADDRESS STA ($30),Y LDA #$FF ;TURN EXCHANGE FLAG ON (= -1) STA $32 CHKEND DEX ;END OF LIST? BNE NXTEL ;NO. FETCH NEXT ELEMENT BIT $32 ;YES. EXCHANGE FLAG STILL OFF? BMI SORT8 ;NO. GO THROUGH LIST AGAIN RTS ;YES. LIST IS NOW ORDERED
The LISP-ish output produced by parsing this with my grammar looks like:
(prog (line (comment ;THIS SUBROUTINE ARRANGES THE 8-BIT ELEMENTS OF A LIST IN ASCENDING)) \n (line (comment ;ORDER. THE STARTING ADDRESS OF THE LIST IS IN LOCATIONS $30 AND)) \n (line (comment ;$31. THE LENGTH OF THE LIST IS IN THE FIRST BYTE OF THE LIST. LOCATION)) \n (line (comment ;$32 IS USED TO HOLD AN EXCHANGE FLAG.)) \n \n (line (instruction (label (name SORT8)) (opcode LDY) (argumentlist (argument (prefix #) (number $00))) (comment ;TURN EXCHANGE FLAG OFF (= 0)))) \n (line (instruction (opcode STY) (argumentlist (argument (number $32))))) \n (line (instruction (opcode LDA) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))) (comment ;FETCH ELEMENT COUNT))) \n (line (instruction (opcode TAX) (comment ; AND PUT IT INTO X))) \n (line (instruction (opcode INY) (comment ;POINT TO FIRST ELEMENT IN LIST))) \n (line (instruction (opcode DEX) (comment ;DECREMENT ELEMENT COUNT))) \n (line (instruction (label (name NXTEL)) (opcode LDA) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))) (comment ;FETCH ELEMENT))) \n (line (instruction (opcode INY))) \n (line (instruction (opcode CMP) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))) (comment ;IS IT LARGER THAN THE NEXT ELEMENT?))) \n (line (instruction (opcode BCC) (argumentlist (argument (name CHKEND))))) \n (line (instruction (opcode BEQ) (argumentlist (argument (name CHKEND))))) \n (line (comment ;YES. EXCHANGE ELEMENTS IN MEMORY)) \n (line (instruction (opcode PHA) (comment ; BY SAVING LOW BYTE ON STACK.))) \n (line (instruction (opcode LDA) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))) (comment ; THEN GET HIGH BYTE AND))) \n (line (instruction (opcode DEY) (comment ; STORE IT AT LOW ADDRESS))) \n (line (instruction (opcode STA) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))))) \n (line (instruction (opcode PLA) (comment ;PULL LOW BYTE FROM STACK))) \n (line (instruction (opcode INY) (comment ; AND STORE IT AT HIGH ADDRESS))) \n (line (instruction (opcode STA) (argumentlist (argument ( (argument (number $30)) )) , (argumentlist (argument (name Y)))))) \n (line (instruction (opcode LDA) (argumentlist (argument (prefix #) (number $FF))) (comment ;TURN EXCHANGE FLAG ON (= -1)))) \n (line (instruction (opcode STA) (argumentlist (argument (number $32))))) \n (line (instruction (label (name CHKEND)) (opcode DEX) (comment ;END OF LIST?))) \n (line (instruction (opcode BNE) (argumentlist (argument (name NXTEL))) (comment ;NO. FETCH NEXT ELEMENT))) \n (line (instruction (opcode BIT) (argumentlist (argument (number $32))) (comment ;YES. EXCHANGE FLAG STILL OFF?))) \n (line (instruction (opcode BMI) (argumentlist (argument (name SORT8))) (comment ;NO. GO THROUGH LIST AGAIN))) \n (line (instruction (opcode RTS) (comment ;YES. LIST IS NOW ORDERED))) \n)
If you have Apple][ code on floppies of your own and you wish to retrieve it, I used a device from here. It worked very well.