Why does tv.spin write to dirb?

mpark

From tv.spin:

        if_nc           mov     dira,t2
        if_nc           mov     dirb,#0
        if_c            mov     dira,#0
        if_c            mov     dirb,t2                 '+18

I thought dirb wasn't connected to anything. Can anyone help me understand what this code is doing?

Rayman

Looks like tv.spin was written when there was still going to be a dirb! I can imagine TV and VGA were the very first drivers made...

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ericball

Chip originally intended to create an enhanced version of the Propeller with 64 I/O pins. Thus the port B registers. When Chip wrote tv.spin, he coded it so it would support pins 32-63 without any additional changes. So the code figures out whether tv_pins[noparse][[/noparse]6..4] maps to pins 0-31 or 32-63 and sets DIRA & DIRB to match.

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mpark

Thanks, makes sense.

Cluso99

Chip coded the spin Interpreter to be compatible with the 64 I/O version. It uses the C flag to determine if it is the lower or upper 32 I/Os. This is defined as "future" in the Propeller Manual.

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