Omnimaga
Calculator Community => TI Calculators => ASM => Topic started by: thepenguin77 on October 21, 2011, 05:53:11 pm
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I was looking through starfox when I saw some code that used DJNZ in an unconventional way, this made me think about how useful it is, so here are some things I just thought of.
On this first one, you're going to say, "well duh," but I bet you don't do it.
Let's say you want to see if A==1:
dec a
jr nz, AWasnt1
You could replace it with this for only 1 extra t-state:
ld b, a
djnz AWasnt1
One benefit of this is that you don't destroy A. Now before you start complaining about that 1 t-state, just remember, if you manage to put your result into B rather than A, this routine becomes 3 t-states faster.
Here is another common code structure that I run into a lot:
ld a, (hl)
dec a
jr nz, not1
...
jr final
not1:
dec a
jr nz, not2
...
jr final
not2:
dec a
jr nz, not3
...
jr final
not3:
dec a
jr nz, not4
...
jr final
not4:
...
final:
Why not replace it with this, which is both faster and smaller (you don't need the jumps if you can leave b at 0):
ld b, (hl)
djnz not1
...
not1:
djnz not2
...
not2:
djnz not3
...
not3:
djnz not4
...
not4:
djnz not5
...
not5:
final:
Finally, this one is a bit more of a stretch, but you can use B as a flag byte, take for instance this byte plotter:
;######################################
;plot a byte
;a = byte
;b = inverse 1 = yes, 0 = no
;de = xy
plotAByte:
ld c, d
ld d, 0
ld h, d
ld l, e
add hl, de
add hl, de
add hl, hl
add hl, hl
srl c
srl c
srl c
ld e, c
add hl, de
ld de, plotSScreen
add hl, de
djnz inversing
or (hl)
ld (hl), a
ret
inversing:
cpl
and (hl)
ld (hl), a
ret
Also, while it's not a new idea today, I have used DJNZ in weird ways before, I typically do it when unpacking bytes from storage. This routine is from zStart and pulls a name out of storage from 6 bytes to 8 bytes. I use B as a bit counter.
pop hl
ld ix, op1+1
ld bc, 8*256+6
ld d, 8
xor a
unpackNameLoop:
rr (hl)
rra
djnz notNextUnpackByte
inc hl
ld b, 8
notNextUnpackByte:
dec c
jr nz, unpackNameLoop
ld c, 6
rra
rra
ld (ix), a
inc ix
xor a
dec d
jr nz, unpackNameLoop
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What
seems cool though
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Why not replace it with this, which is both faster and smaller (you don't need the jumps if you can leave b at 0):
ld b, (hl)
djnz not1
...
not1:
djnz not2
...
not2:
djnz not3
...
not3:
djnz not4
...
not4:
djnz not5
...
not5:
final:
Wow! That's great stuff! :D
Plus you can keep your original value unchanged in A.
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I use djnz to jump forward very often, actually, and it is a trick I figured out and exploited when I created my first asm library all the way up to BatLib :) I even made a note of it in my opcode chart:
DJNZ * decrements B. If B is not 0, it pretends to be a jr. If B is 0, the code continues. This is mostly used for loops because it uses only 2 bytes, so * is usually negative. Because of this, people seem to forget that you can jump forward with this. :P
EDIT: and I use it for things like selecting a sprite method to draw with, rectangle method, pixel plotting method, et cetera.
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Another DJNZ hack is that you can use DJNZ $+2 to decrease B without affecting any flags. It's 4-9 cycles slower than normal DEC B, though.
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Brilliant
(And useless in 99% of cases)
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It is one of those things that may not be used for a long time, but when it happens, somebody is going to feel special XD
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That looks very nice, the djnz bundles dec, jr, nz nicely, saving space and adding efficiency. Could be useful, +1.
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It's amazing how creative you all are. For the djnz list i usually just use a jump table and maybe push a return value onto the stack, but that looks like it'd be smaller (if not faster), though you'd have to be careful with 'b'.
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ld b,0
for_loop:
;stuff here
djnz for_loop
;will repeat 256 times
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I think the best DJNZ is to ONLY use DJNZ :D I was reading about Single Instruction Set Computers (http://en.wikipedia.org/wiki/One_instruction_set_computer) and learned that using only DJNZ you can produce a turring complete language!
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I think the best DJNZ is to ONLY use DJNZ :D I was reading about Single Instruction Set Computers (http://en.wikipedia.org/wiki/One_instruction_set_computer) and learned that using only DJNZ you can produce a turring complete language!
Mind = Blown
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What does turring complete language mean? especially the turring part.
Do you mean Turing, as for example, the Turing test?
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Turing Completeness (http://en.wikipedia.org/wiki/Turing_completeness) is a computer science term referring to the ability of a language to complete problems. If a language is Turing Complete, it is able to solve any problem a Turing computer could solve.