# Programming Basics

## Opcodes and their operands

To be written.

## Registers

The 6502 processor has six registers. They are 8-bit, with the exception of the Program Counter, which is 16-bit.

Accumulator(A) - The accumulator can read and write to memory. It is used for arithmetic and logic.

X Index - The x index is can read and write to memory. It is used primarily as a counter in loops, or for addressing memory, but can also temporarily store data like the accumulator.

Y Index - Much like the x index, however they are not completely interchangeable. Some operations are only available for each register.

Flag(P) - Seven bits that represent the status of the processor.

Stack Pointer(S) - The stack pointer hold the address to the current location on the stack. The stack is a way to store data by pushing or popping data to and from a section of memory.

Program Counter(PC) - This register keeps up with the processor's current location in a program.

## The stack

*Main article: Stack*

## Math operations

### Simple operations

#### Addition and subtraction

*To be written.*

#### Bitwise (factor of 2) multiplication and division

To multiply the value in A by two, use the instruction ASL A.

To divide the value in A by two, use the instruction LSR A.

*To be written.*

### Complex operations

#### Multiplication of arbitrary numbers

The following routine multiplies two unsigned 16-bit numbers, and returns an unsigned 32-bit value.

mulplr = $c0 ; ZP location = $c0 partial = mulplr+2 ; ZP location = $c2 mulcnd = partial+2 ; ZP location = $c4 _usmul: pha tya pha _usmul_1: ldy #$10 ; Setup for 16-bit multiply _usmul_2: lda mulplr ; Is low order bit set? lsr a bcc _usmul_4 clc ; Low order bit set -- add mulcnd to partial product lda partial adc mulcnd sta partial lda partial+1 adc mulcnd+1 sta partial+1 ; ; Shift result into mulplr and get the next bit of the multiplier into the low order bit of mulplr. ; _usmul_4: ror partial+1 ror partial ror mulplr+1 ror mulplr dey bne _usmul_2 pla tay pla rts

Here's an example of the above `_usmul` routine in action, which multiplies 340*268:

lda #<340 ; Low byte of 16-bit decimal value 340 (value: $54) sta mulplr lda #>340 ; High byte of 16-bit decimal value 340 (value: $01) (makes $0154) sta mulplr+1 lda #<268 ; Low byte of 16-bit decimal value 268 (value: $0C) sta mulcnd lda #>268 ; High byte of 16-bit decimal value 268 (value: $01) (makes $010C) sta mulcnd+1 lda #0 ; Must be set to zero (0)! sta partial sta partial+1 jsr _usmul ; Perform multiplication ; ; RESULTS ; mulplr = Low byte of lower word (bits 0 through 7) ; mulplr+1 = High byte of lower word (bits 8 through 15) ; partial = Low byte of upper word (bits 16 through 23) ; partial+1 = High byte of upper word (bits 24 through 31) ;

#### Division of arbitrary numbers

To be written.

#### Floating-point numbers

To be written.

#### Gaming: keeping score

To be written.

If you keep score in a binary number, you must convert it to a sequence of digits before displaying it. The article 16-bit BCD lists a subroutine to do this.

## Making simple sounds

To be written.

## Controller input

To be written.

## Graphics (should be covered elsewhere!)

## "Hello, world!" program

Since the NES can't easily do something like `printf()`

(or `echo`

for those familiar with scripting), one of the easiest ways to test code is to output some audio. Something along the lines of...

reset: lda #$01 ; square 1 sta $4015 lda #$08 ; period low sta $4002 lda #$02 ; period high sta $4003 lda #$bf ; volume sta $4000 forever: jmp forever