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Forums > C64 Coding > fastest or smallest 'switch' statment
2015-09-05 15:02
mhindsbo

Registered: Dec 2014
Posts: 51
fastest or smallest 'switch' statment

I use the following code a lot to switch between two values and was wondering what others do and if there is a faster, smaller or simply more elegant way someone has come up with.
      lda #value1    ; default value: AR = value1
      ldy switch     ; get value of switch in YR 
      cpy #case1     ; compare switch 
      beq @cont
      lda #value2    ; if switch != #case1 then AR = value2

@cont ...            ; AR = value1/value2 depending on switch
2015-09-05 15:11
iAN CooG

Registered: May 2002
Posts: 3194
a table, use y as index
ldy switch
lda table,y

table
byte value1, value2, etc
2015-09-05 15:51
mhindsbo

Registered: Dec 2014
Posts: 51
Absolutely ... That is probably the most elegant, especially for multiple values.

But what about the case where you have two switch values that are not sequential. E.g. two different screen address (hi byte), where switch value is not 0,1,2,3,... But e.g. $c0 and $d0.

Any elegant alternatives in that specific case?
2015-09-05 16:31
iAN CooG

Registered: May 2002
Posts: 3194
if all you have is a 8bit index and want to return a 8bit value, a $100 bytes table is always the fastest solution, just place the values where needed, unless you don't have a free page ($100 bytes) in memory for this lookup.
But if you just need to select 2 values, a cmp is enough..
2015-09-05 16:57
Fungus

Registered: Sep 2002
Posts: 686
Just 2 values?

mod1
lda value
eor #$xx
sta mod1+1

where xx is the eor to get the other value you want to toggle between.
2015-09-05 18:43
soci

Registered: Sep 2003
Posts: 480
Fungus: There was at least a '#' missing.
mod1 lda #value1
     eor #value1 ^ value2
     sta mod1 + 1

But this is not what he wants to do.

For $c0 and $d0 only a shorter table could do it fast.
      ldy switch
      lda table - $c0,y

table .fill $d0 - $c0, value1
      .byte value2
2015-09-05 19:46
lft

Registered: Jul 2007
Posts: 369
Quoting iAN CooG
a table, use y as index
ldy switch
lda table,y

table
byte value1, value2, etc


Sometimes this could be faster:

switch = * + 1
lda table
2015-09-05 20:23
mhindsbo

Registered: Dec 2014
Posts: 51
Thanks for all the input. good stuff. I dont have $100 to spend on a table unfortunately.

I use it in a number of functions where the switch values are different (but all to switch between two states)... so it would lead to multiple tables.

The 'cmp' is probably the best in this use case, but it just felt so inelegant using it all the time ;-)
2015-09-05 20:50
Oswald

Registered: Apr 2002
Posts: 5094
can you cite a real world example where you need to use this ? maybe it can be done algorithmically better.
2015-09-05 23:20
mhindsbo

Registered: Dec 2014
Posts: 51
In my game I have a number of objects (enemies, bullets, ...) and each has a specific identifier (0-255). In many of the object routines I check for a specific object or a specific state and take one of two actions or set one of two parameters.

Hope this helps explain it. Its always a balance of speed and size for a game. Tables can be obvious as can unrolled code or specific code for specific objects ... but with level graphics, music, etc. I find myself often in a compromise between the two.

E.g. the table lookup is faster, but cant do too many $100 tables. Specific code for each object is faster, but similar ends up eating up memory quickly.

Hope that gives some context. I am decently happy with what I mentioned originally in terms of compromise to switch between to values depending on a parameter ... but just thought I would seek some inspiration as well.

Thanks to everyone who chimed in!
2015-09-05 23:29
soci

Registered: Sep 2003
Posts: 480
Quoting mhindsbo
The 'cmp' is probably the best in this use case, but it just felt so inelegant using it all the time ;-)

Well, then use the one below for a change ;)
      lda switch         ; get value of switch in AR.
      eor #case1         ; compare switch.
      beq @cont
      lda #value2^value1 ; if switch != #case1 then AR = value2
@cont eor #value1        ; default value: AR = value1
                         ; AR = value1/value2 depending on switch

It's not faster or shorter, but it's only using the accumulator and does not destroy the carry.
 
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