Those slow ZBIT instructions

[King Harold]

As we all know ZBIT instructions on registers take 8 T-states.
Boolean instructions with an immediate as operant take 7 T-states.
So in some cases (when working with a) you could save 1 T-state.

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bit n,a

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and %[7-n zero's]1[n zero's]
jp pe,BitReset
;when you're here the bit is Set

this code does modify the contents of A so it is mostly useless

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res n,a

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and %[7-n one's]0[n one's]

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set n,a

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or %[7-n zero's]1[n one's]

and something new, one could call it 'cbit', a pitty it doesn't exist on the z80. it would complement the n-th bit.

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cbit n,a

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xor %[7-n zero's]1[n one's]

correct me where I'm wrong (probably at the pe/po part..)

although most of us probably already knew this, I just thought I'd bring it up, so that those who didn't know it yet would not waste 1 T-state.

[benryves]

I'm not sure why you use pe; you can just use the z flag (and so also have relative jumps if need be).
You could set up a macro for these, I suppose;
(Destructive bit, fast reset, fast set).

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#define dbit(n)and 1<<n
#define fres(n)and ~(1<<n)
#define fset(n)or 1<<n
#define cbit(n)xor 1<<n

[Jim e]

pe/po? heard of z and nz.

It depends on the situation for which to use, AND could be used in place of bit when the rest of the contents don't matter. Bit is prefered when you may need to test different bits out of order but consecutively on the same byte.


If you need to test bits in order and consecutively you could

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   rrca
   jp c,set0
   rrca
   jp c,set1

which is accomplishes

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   bit 0,a
   jp nz,set0
   bit 1,a
   jp nz,set1

or instead of

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   bit 7,a
   jp nz,set7

you could

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   or a
   jp m,set7

and if need be you could test 2 bits of a register other than a at the same time, for example

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   bit 7,b
   jp nz,set7
   bit 6,b
   jp nz,set6

could be

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   rlc b
   jp c,set7
   jp m,set6

[King Harold]

That last one looks a bit scary to me, but I think I've used most others
But what's against pe/po?

btw, that 1<<n is extremely clever Ben