Programming Basics: Difference between revisions
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==== Multiplication of arbitrary numbers ==== | ==== Multiplication of arbitrary numbers ==== | ||
The following routine multiplies two unsigned 16-bit numbers, and returns an unsigned 32-bit value. | The following routine multiplies two unsigned 16-bit numbers, and returns an unsigned 32-bit value. | ||
< | <source lang="6502tasm"> | ||
mulplr = $c0 ; ZP location = $c0 | mulplr = $c0 ; ZP location = $c0 | ||
partial = mulplr+2 ; ZP location = $c2 | partial = mulplr+2 ; ZP location = $c2 | ||
| Line 67: | Line 67: | ||
pla | pla | ||
rts | rts | ||
</ | </source> | ||
Here's an example of the above <tt>_usmul</tt> routine in action, which multiplies 340*268: | Here's an example of the above <tt>_usmul</tt> routine in action, which multiplies 340*268: | ||
< | <source lang="6502tasm"> | ||
lda #<340 ; Low byte of 16-bit decimal value 340 (value: $54) | lda #<340 ; Low byte of 16-bit decimal value 340 (value: $54) | ||
sta mulplr | sta mulplr | ||
| Line 91: | Line 91: | ||
; partial+1 = High byte of upper word (bits 24 through 31) | ; partial+1 = High byte of upper word (bits 24 through 31) | ||
; | ; | ||
</ | </source> | ||
==== Division of arbitrary numbers ==== | ==== Division of arbitrary numbers ==== | ||
| Line 120: | Line 120: | ||
Since the NES can't easily do something like <code>printf()</code> (or <code>echo</code> for those familiar with scripting), one of the easiest ways to test code is to output some audio. Something along the lines of... | Since the NES can't easily do something like <code>printf()</code> (or <code>echo</code> for those familiar with scripting), one of the easiest ways to test code is to output some audio. Something along the lines of... | ||
< | <source lang="6502tasm"> | ||
reset: | reset: | ||
lda #$01 ; square 1 | lda #$01 ; square 1 | ||
| Line 132: | Line 132: | ||
forever: | forever: | ||
jmp forever | jmp forever | ||
</ | </source> | ||
Revision as of 09:20, 24 August 2013
Opcodes and their operands
To be written.
Registers
To be written.
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. <source lang="6502tasm"> 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
</source>
Here's an example of the above _usmul routine in action, which multiplies 340*268:
<source lang="6502tasm">
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)
</source>
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...
<source lang="6502tasm"> 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
</source>
