I want to have a routine that decreases a 16 bit value, and gives a flag when 0 is reached.
Example: an endboss has 400 life-points, whenever he gets hit he looses 1 life.

I was thinking about this, but it doesnt look very good/fast:

at startup:

LD HL,400
LD (Bosslife),HL

in game when boss is hit:

XOR A ;reset carry

LD HL,(Bosslife)
LD DE,1
SBC HL,DE
LD (Bosslife),HL
JP Z,BossDies


Does anyone have a faster/better way to do this ?
thank you !

Quote:

I want to have a routine that decreases a 16 bit value, and gives a flag when 0 is reached. Example: an endboss has 400 life-points, whenever he gets hit he looses 1 life. I was thinking about this, but it doesnt look very good/fast: at startup: LD HL,400 LD (Bosslife),HL in game when boss is hit: XOR A ;reset carry LD HL,(Bosslife) LD DE,1 SBC HL,DE LD (Bosslife),HL JP Z,BossDies Does anyone have a faster/better way to do this ? thank you !

What about:

LD HL,(Bosslife)
DEC HL
LD (Bosslife),HL
LD A,H
OR L
JP Z,BossDies

why change that routine? this way you can subtract faster by increasing DE... for example depending on the strength of your weapon.

I came up with the following. Note that you have to add 256 to the boss life (ld hl,400+256).

bosslifecheck
  ld hl,Bosslife
  dec (hl)
  ret nz
  inc hl
  dec (hl)
  ret

This routine is so fast you could call it a few times if you are fireing with a stonger weapon

that routine only decreases bosslife by one... that 'll take long for the player to kill the basterd
the initial routine will do just fine, but dunno if Z flag is set when substraction crosses 0...

Here is a routine that supports stonger 'weapons'.

in: b  =  strenght
    hl -> Bosslife (pointer)
out: carry if zero

lifecheck
  ld a,(hl)
  sub b
  ld (hl),a
  jr z,.hup
  ret nc
.hup
  xor a
  inc hl
  dec (hl)
  jp nz,.hup2
  scf
.hup2
  ret

use it the following way

  ld hl,Bosslife
  ld b,5
  call lifecheck
  jp c,BossDead

Hmmm does it really work how it is supposed to...
Let's shorten the code a bit:

in: b  =  strenght
    hl -> Bosslife (pointer)
out: carry if zero

lifecheck
  ld a,(hl)
  sub b
  ld (hl),a
  ret nc
  inc hl
  dec (hl)
  ret z
  ccf
  ret

This should work fine too. Just shoot one more bullet (In case you reach exactly zero that is)

Quote:

What about: LD HL,(Bosslife) DEC HL LD (Bosslife),HL LD A,H OR L JP Z,BossDies

Doesnt that give a zero flag when H=L instead of when HL=0 ??

Quote:

I came up with the following. Note that you have to add 256 to the boss life (ld hl,400+256). bosslifecheck ld hl,Bosslife dec (hl) ret nz inc hl dec (hl) ret

this one is great (if it works). I dont quite get it though. DEC (HL) doesnt set any flags... ???

Quote:

Hmmm does it really work how it is supposed to... Let's shorten the code a bit: in: b = strenght hl -> Bosslife (pointer) out: carry if zero lifecheck ld a,(hl) sub b ld (hl),a ret nc inc hl dec (hl) ret z ccf ret This should work fine too. Just shoot one more bullet (In case you reach exactly zero that is)

i get it, that looks good, Im gonna give it a try !! thanx

You're welcome, just remember to add 256 to the life meter.
And you could lose all those calls and rets to speed things up.

Quote:

Doesnt that give a zero flag when H=L instead of when HL=0 ?

ld a,h / cp l gives z if h=l. ld a,h / or l is valid for testing hl=0

Quote:

this one is great (if it works). I dont quite get it though. DEC (HL) doesnt set any flags... ???

It does set flags just no carry flag.

Great @@ gracias

Hint: when you want to subtract a constant value from HL like this:

ld de,32
or a
sbc hl,de

... replace with addition of negatief constant:

ld de,-32
add hl,de

This will save you 1 instruction.

Quote:

This will save you 1 instruction.

Thus testing for smaller than zero like:

bit 7,h
jp nz,blablabla

(note that hl must be smaller than 32768 to begin with)

instead of:

jp c,blablabla

But still a very usefull hint

Anvill suggested adding a negative value rather than subtracting a positive value, but GhostwriterP sees a problem:

Quote:

Quote: This will save you 1 instruction. Thus testing for smaller than zero like: bit 7,h jp nz,blablabla (note that hl must be smaller than 32768 to begin with) instead of: jp c,blablabla But still a very usefull hint

With Anvills trick you can just use
jp nc,blablabla
and hl could start at over 32768.

Such high values for life counters may not be a very good idea though; one can easily create monsters that take too much time to get killed or one can introduce the suggestion of an accuracy that does not really make sense.