Identity table: Difference between revisions
From NESdev Wiki
Jump to navigationJump to search
(addition/subtraction by 2 tables) |
(fix reference) |
||
| Line 1: | Line 1: | ||
An identity table is simply a table with bytes 0 to 255 in ascending order. The value at any index is equal to the index: | An identity table is simply a table with bytes 0 to 255 in ascending order, usually in PRG ROM. The value at any index is equal to the index: | ||
identity_table: | identity_table: | ||
| Line 70: | Line 70: | ||
All the instructions above take 3 bytes and 4 CPU cycles, assuming the table is page-aligned (starts at <tt>$xx00</tt>) and not on zero page. (Crossing a page boundary causes slowdown with indexed addressing modes.) | All the instructions above take 3 bytes and 4 CPU cycles, assuming the table is page-aligned (starts at <tt>$xx00</tt>) and not on zero page. (Crossing a page boundary causes slowdown with indexed addressing modes.) | ||
<ref>[http://forums.nesdev.org/viewtopic.php?f=2&t=3682 NESDev forum - "6502 ASM trick" - post by tokumaru]</ref> | |||
== Constant offsets with two identity tables == | == Constant offsets with two identity tables == | ||
| Line 84: | Line 86: | ||
* <tt>adc identity_table2-''constant'',x</tt> is equivalent to <tt>adc x-''constant''</tt> where ''constant'' = 1-255 | * <tt>adc identity_table2-''constant'',x</tt> is equivalent to <tt>adc x-''constant''</tt> where ''constant'' = 1-255 | ||
<ref> | <ref>[https://forums.nesdev.org/viewtopic.php?p=127081#p127081 NESDev forum - "6502 ASM trick" - post by Jarhmander]</ref> | ||
== See also == | == See also == | ||
| Line 93: | Line 95: | ||
== References == | == References == | ||
<references/> | |||
Latest revision as of 00:57, 7 February 2023
An identity table is simply a table with bytes 0 to 255 in ascending order, usually in PRG ROM. The value at any index is equal to the index:
identity_table:
.byte $00,$01,$02,$03,$04,$05,$06,$07,$08,$09,$0a,$0b,$0c,$0d,$0e,$0f
.byte $10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$1a,$1b,$1c,$1d,$1e,$1f
.byte $20,$21,$22,$23,$24,$25,$26,$27,$28,$29,$2a,$2b,$2c,$2d,$2e,$2f
.byte $30,$31,$32,$33,$34,$35,$36,$37,$38,$39,$3a,$3b,$3c,$3d,$3e,$3f
.byte $40,$41,$42,$43,$44,$45,$46,$47,$48,$49,$4a,$4b,$4c,$4d,$4e,$4f
.byte $50,$51,$52,$53,$54,$55,$56,$57,$58,$59,$5a,$5b,$5c,$5d,$5e,$5f
.byte $60,$61,$62,$63,$64,$65,$66,$67,$68,$69,$6a,$6b,$6c,$6d,$6e,$6f
.byte $70,$71,$72,$73,$74,$75,$76,$77,$78,$79,$7a,$7b,$7c,$7d,$7e,$7f
.byte $80,$81,$82,$83,$84,$85,$86,$87,$88,$89,$8a,$8b,$8c,$8d,$8e,$8f
.byte $90,$91,$92,$93,$94,$95,$96,$97,$98,$99,$9a,$9b,$9c,$9d,$9e,$9f
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$a8,$a9,$aa,$ab,$ac,$ad,$ae,$af
.byte $b0,$b1,$b2,$b3,$b4,$b5,$b6,$b7,$b8,$b9,$ba,$bb,$bc,$bd,$be,$bf
.byte $c0,$c1,$c2,$c3,$c4,$c5,$c6,$c7,$c8,$c9,$ca,$cb,$cc,$cd,$ce,$cf
.byte $d0,$d1,$d2,$d3,$d4,$d5,$d6,$d7,$d8,$d9,$da,$db,$dc,$dd,$de,$df
.byte $e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7,$e8,$e9,$ea,$eb,$ec,$ed,$ee,$ef
.byte $f0,$f1,$f2,$f3,$f4,$f5,$f6,$f7,$f8,$f9,$fa,$fb,$fc,$fd,$fe,$ff
Using the identity table, new assembly instructions can be synthesized as follows:
| ldx identity_table,y | tyx |
| ldy identity_table,x | txy |
| and identity_table,x | and X |
| and identity_table,y | and Y |
| ora identity_table,x | ora X |
| ora identity_table,y | ora Y |
| eor identity_table,x | eor X |
| eor identity_table,y | eor Y |
| adc identity_table,x | adc X |
| adc identity_table,y | adc Y |
| sbc identity_table,x | sbc X |
| sbc identity_table,y | sbc Y |
| cmp identity_table,x | cmp X |
| cmp identity_table,y | cmp Y |
| bit identity_table+constant | bit #constant |
All the instructions above take 3 bytes and 4 CPU cycles, assuming the table is page-aligned (starts at $xx00) and not on zero page. (Crossing a page boundary causes slowdown with indexed addressing modes.)
Constant offsets with two identity tables
By defining another identity table immediately after the first one, the values of X and Y can be offset by a constant, at the cost of 255 more ROM bytes:
identity_table1:
.byte $00, $01, ..., $ff ; as above
identity_table2:
.byte $00, $01, ..., $fe ; $ff not needed
For example:
- adc identity_table1+constant,x is equivalent to adc x+constant where constant = 0-255
- adc identity_table2-constant,x is equivalent to adc x-constant where constant = 1-255
