ASM Question - Not understanding out (#0xff),a and PIO + PSG

[Shining]

I have here this time-critical routine to play samples on the psg which I don't completely undestand. But I want to:


A little code-snippet:
ld      de,#0xb6f6


later on:

ld      a,e         ; 4
out     (#0xff),a     ; Send &f6 to port &f6 (Select Register)


Why does this output on port 0xF6 ?




later on:

ld      a,d         ; 4
out     (#0xff),a     ; 12 Send &b6 to port &f6 (Write Register)



And why does this output on port 0xF6 ?

[arnoldemu]

I have here this time-critical routine to play samples on the psg which I don't completely undestand. But I want to:


A little code-snippet:
ld      de,#0xb6f6


later on:

ld      a,e         ; 4
out     (#0xff),a     ; Send &f6 to port &f6 (Select Register)


Why does this output on port 0xF6 ?




later on:

ld      a,d         ; 4
out     (#0xff),a     ; 12 Send &b6 to port &f6 (Write Register)



And why does this output on port 0xF6 ?

out (nn),a

register A is put into i/o address bits 15-8. nn is put into i/o address bits 7..0.

and register A data is written to the port.

Code Select Expand


ld      a,e         ; 4
out     (#0xff),a     ; Send &f6 to port &f6 (Select Register)


This is the same as out &F6FF,&F6. (here A = f6)

port f6 bits 7 and 6 are PSG "function", this is from the data written to &f6xx.
00xxxxxx = inactive,
11xxxxxx = register select,
01xxxxxx = read data,
10xxxxxxx = write data.

bits 3..0 are keyboard line, other bits for cassette.

Other OUT (n),A is out &b6ff,&b6.
with &b6 as data 10110110. bit 7=1,bit6 = 0, which is write data to psg.

decoding of both of these ports:

1111011011111111‬ <-f6ff
1011011011111111 <-b6ff

port decoding for 8255:

%xxxx0x10 xxxxxxxx

both have this in common. So both will access 8255.

But b6ff clashes with crtc because it decodes I/O address with:
%x0xxxxxx xxxxxxxx

Thankfully the other bits mean that crtc is selected for reading and it's doing a write. Shouldn't be a problem.

[GUNHED]

Don't use port B6, this may work on emulators, but it's dirty coding on a real CPC.

[Shining]

Thanks for your explanations ! I was aware what the code did but not why. I did not know that the bits in a were the high-byte of the address.


Because the code is for this Viterbi search 3-channel-digi-routine like (https://github.com/maxim-zhao/pcmenc), timing is very important.
But like the out is used in this algorithm, the out turns always the tape-relais on. I wanted to understand why the out worked and how to possibly find a solution (taking the same amout of cycles) where I can change the value for the port.


The original algorithm loads registers outside the time-critical part:



ld      b,#0xf4
ld      de,#0xb6f6
ld      hl,#0x3608


And then during the time-critical part ist does:

ld      a,e         ; 4
out     (#0xff),a     ; 12 Send &f6 to port &f6 (Select Register)
ld      c,l         ; 4
out     (c),c       ; 16 Send &08 to port &f4 (Channel A)
ld      a,h         ; 4
out     (#0xff),a     ; 12 Send &36 to port &f6 (Inactive State)


Which are:
out &b6ff,&b6
out &F4xx,&08
out &36FF,&36 (which also adresses the pio. Again thanks for your explanation)


When I'm right and I replace &F6 through &E6 and &36 through &16, also the PIO is adressed but the relais remains off.
I'm now (thanks gunhed) aware that this also addresses other hardware, but is this possible ? I do not find an "undirty" replacement of the code, using exactly the same amount of cycles.

[GUNHED]

Ok, this dirty coding saves few us, but is that needed?


I would suggest to use


LD BC,&XXXX
OUT (C),C


This way still should be quick enough.