Assembly (6502)

Assembly is an extremely low-level human-readable language that has a strong relation between the code and the corresponding machine code.

The Motorola 6502 is a microcontroller based on their 6800 series, but simplified, less expensive, and faster.

Memory Locations

`$0000 - $00ff` - Zero Page

This stores commonly used game variables as they can be accessed with only two digit addresses (see addressing modes).

`$0200 - $05ff` - Display

This stores values to be rendered on the display. Each row of the screen is represented by $20 values stored inline, rendering left to right. e.g. the first row of the screen is held in $0200 - $021f; the second row is held in $0220 - $023f.

Syntax

Literals

Type	Syntax	Notes
Binary	`%nnnnnnnn`	`n` is a binary digit. Trailing numbers can extend
Hexadecimal	`$xy`	`xy` are hex digits. Trailing numbers can extend
Decimal	`xy`	`xy` are decimal digits. Trailing numbers can extend

Flags

Flag	Name	Notes
C	Carry Flag	The carry flag is set if the last operation caused an overflow from bit 7 of the result or an underflow from bit 0. This condition is set during arithmetic, comparison and during logical shifts (or explicitly).
Z	Zero Flag	The zero flag is set if the result of the last operation as was zero.
I	Interrupt Disable	The interrupt disable flag is set if the program has executed a `SEI` instruction. While this flag is set the processor will not respond to interrupts from devices until it is cleared by a `CLI` instruction.
D	Decimal Mode Flag	While the decimal mode flag is set the processor will obey the rules of Binary Coded Decimal (BCD) arithmetic during addition and subtraction. The flag can be explicitly set using `SED` and cleared with `CLD`.
B	Break Command	The break command bit is set when a `BRK` instruction has been executed and an interrupt has been generated to process it.
V	Overflow Flag	The overflow flag is set during arithmetic operations if the result has yielded an invalid 2's complement result (e.g. adding to positive numbers and ending up with a negative result: 64 + 64 => -128). It is determined by looking at the carry between bits 6 and 7 and between bit 7 and the carry flag.
N	Negative Flag	The negative flag is set if the result of the last operation had bit 7 set to a one.

Addressing Modes

These will all use STA but can be used with any command that uses an address.

Type	Syntax	Effect
Absolute	`LDA $c000`	Load the value at this full memory location
Zero page	`LDA $c0`	Load the value at `$00c0`, `c0` within the zero page
Absolute, X	`LDA $c000,X`	Load the value at `$c000` + the value in register X (e.g. if X is `$01`, load value at memory location `$c001`)
Zero page, X	`LDA $c0,X`	Load the value at `$00c0` + the value in register X (e.g. if X is `$01`, load value at memory location `$c001`)
Immediate	`LDA #$c0`	Load value `$c0`
Relative	`$c0` or `label`	Go `$c0` bytes forward/backward (to get to label position). Used with branching and jump commands (see below)
Implicit	`INX`	Do what instruction implies (no args)
Indirect	`LDA ($c000)`	Load value at memory address `$c000` and the following byte, with the first address being the least significant byte, and the following address the most significant byte (e.g. in this example, if `$00c0` holds `01` and `$00c1` holds `0f`, it would dereference to `$0f01`).
Indexed Indirect	`LDA ($c0,X)`	Load value at the address (`$00c0` + `X`), with the resulting first address being the least significant byte, and the following address the most significant byte (e.g. in this example, if `X` was `01`, we would look at the address `$00c1`. If `$00c1` holds `01` and `$00c2` holds `0f`, it would dereference to `$0f01`). NOTE: Can only be used with X
Indirect Indexed	`LDA ($c0),Y`	Load value at the address found at `$00c0` plus the value of `Y`, with the resulting first address being the least significant byte, and the following address the most significant byte (e.g. in this example, lets say `Y` was `01`. We would look at the address `$00c0`. If `$00c0` holds `01` and `$00c1` holds `0f`, it would dereference to `$0f01` plus the value of `Y`, resulting in `$0f02`).

Commands

A value refers to a number prefaced by a #, whereas an address/location refers to a hexadecimal address.

Command	Argument(s)	Effect
`BRK`		Break out of the program
`org $n`	`$n`: Number	Set program origin to `$n`
`LDA n`	`n`: value or memory address/location where value is stored	Load `n` into register A
`STA n`	`n`: memory address/location	Store value in register A at `n`
`TAX`		Transfer value in A to X
`INX` / `DEX`		Increment/Decrement value in X by 1
`ADC n`	`n`: value or memory address/location where value is stored	Add `n` with carry to the A register

Flag Commands

These commands are used solely for the purpose of changing and setting flags.

Command	Argument(s)	Effect
`CMP n` / `CPX n` / `CPY n`	n: value or memory address/location where value is stored	Compare value at A/X/Y to n. If equal, set `Z` flag to 1; else, set `Z` flag to 0. Note: behind the scenes, these instructions are doing subtraction.
`SEI` / `CLI`		Set/Clear Interrupt Disable

Branching

label: A label that must be within 256 bytes of invocation

Any label can be substituted for a direct memory address.

Command	Name	Effect
`BEQ label`	Branch on EQual	If `Z` flag is 1 (comparison was equal), go to label
`BNE label`	Branch on Not Equal	If `Z` flag is 0 (comparison was not equal), go to label
`BCS label`	Branch on Carry Set	If carry flag is 1 (set), go to label
`BCC label`	Branch on Carry Clear	If carry flag is 0 (not set), go to label
`JMP label`	Unconditional Jump	Jump directly to label
`JSR label` / `RTS`	Jump to SubRoutine / ReTurn from Subroutine	Jump directly to label and push current address - 1 to the stack. This will be retrieved via the return statement.
`BVS label`	Branch on oVerflow Set	If `V` flag is set, or bit 6 is set when using `BIT`, go to label
`BVC label`	Branch on oVerflow Clear	If `V` flag is not set, or bit 6 is not set when using `BIT`, go to label
`BMI label`	Branch on MInus	If `N` flag is set, or bit 7 is set when using `BIT`, go to label
`BPL label`	Branch on PLus	If `N` flag is not set, or bit 7 is not set when using `BIT`, go to label

Stack

Command	Name	Effect
`PHA`	PusH Accumulator	Push the accumulator onto the top of the stack
`PLA`	PulL (Pop) Accumulator	Pop the accumulator off the top of the stack

Bitwise Operators

Command	Parameters	Effect
`AND n`	`n`: Value or address	Bitwise AND of value of `n` or value at address `n` against the accumulator register
`LSR [n]`	`n`: Value or address (if omitted, default to accumulator)	Bitwise shift right

References

IDE and Debugger:

https://8bitworkshop.com/v3.9.0/?platform=vcs&file=examples%2Fhello.a

Instructions and Flags Reference:

Examples:

Incoming Links

Last modified: 202401040446