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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
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mhindsbo
Registered: Dec 2014
Posts: 51 |
nice ... I like it :-) |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
Depending on the values you wish as result, there's also the option to go like this:
cmp #switch
arr #$00
this would work if you need $80 and $00 as resulting values, so it would be interesting what the input and output values are, for some, there can be nice code constructs to generate the resulting values with a few mnemonics and without a branch.
Also, if the result would be just 2 values, the table approach woud just need 2 values strayed into the code somewhere, should be bearable in regards of memory footprint.
Another example that sets bit 4 depending on carry and toggling bit 5 on every call:
cmp #switch
and #$ef ;clear bit 4
adc #$20 ;toggle bit 5 and set bit 4 depending on carry -> adc #$20/21
ora #$0f ;set all lower bits again (might be omitted)
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lft
Registered: Jul 2007
Posts: 369 |
Quoting mhindsboIn 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.
This will again depend on circumstances, but sometimes it is useful to encode information about the objects in a flag table. If you have 256 objects, and you need to switch one way or the other depending on whether an object is edible, held, a key, dangerous etc., then you might encode that as flags in a table. Some flags will be static, and some will change during gameplay. Then you have a one-page table that tracks eight such flags per object.
; object number in y
lda flags1,y
and #$40 ; is this a bullet?
beq ...
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Frantic
Registered: Mar 2003
Posts: 1648 |
http://codebase64.org/doku.php?id=base:dispatch_on_a_byte |
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mhindsbo
Registered: Dec 2014
Posts: 51 |
thanks all! some good input. I'm glad I asked. |
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Fred
Registered: Feb 2003
Posts: 285 |
There is also a way of doing this without using any branch instruction or jump table.
Certainly not the fastest and also not the smallest code on a 6510 CPU:
lda switch
cmp #case1
php
pla
lsr
and #$01
eor #$ff
adc #$00
and #value1 - value2
clc
adc #value2 This can be optimized a bit by using an undocumented instruction:
lda switch
cmp #case1
php
pla
asr #$02
eor #$ff
adc #$01
and #value1 - value2
clc
adc #value2
For the 6510 cpu it doesn't matter a lot to avoid branches. For e.g. Intel based CPUs it does matter in certain cases. Some compilers will optimize the following:
if (condition) {
value = 12;
} else {
value = 34;
}
into this:
value = - Integer(condition) and (12 - 34) + 34;
Btw, the example of Bitbreaker using ARR #$00 will not work since the zero flag isn't taken into account. |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
Quoting FredBtw, the example of Bitbreaker using ARR #$00 will not work since the zero flag isn't taken into account.
As said, depending on the values: If value1 < value2 then carry will be either set on equal or cleared on not equal, just fair if you need to differ two cases with static values only. |
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Fred
Registered: Feb 2003
Posts: 285 |
True. With the right values it is a nice and short solution. |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
Quoting Fred
lda switch
cmp #case1
php
pla
asr #$02
eor #$ff
adc #$01
and #value1 - value2
clc
adc #value2
value = - Integer(condition) and (12 - 34) + 34;
Wouldn't this also be the same?
lda switch
cmp #case1
php
pla
asr #$02
sbc #$00 ;results in either $00 or $ff
and #value2 - value1
clc
adc #value1
Where we take:
value = (Integer(condition) - 1) and (34 - 12) + 12;
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Fred
Registered: Feb 2003
Posts: 285 |
Nice one.
Another optimization that can be done is when the AND uses a value of less than 128, the ANC instruction can be used instead so that the CLC afterwards can be removed. |
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