OZ_Di, Disable M1 interrupts

CALL $0051

IN:

            -
OUT:
            A = old I register
            Fc = old interrupt status (Fc = 1 disabled; Fc = 0 enabled)

Registers changed after return:
            ..BCDEHL/IXIY same
            AF....../.... different

Notes:

It is very important to use this call and not use the Z80 interrupt instructions because of a hardware bug in the Z80, which allows for interrupts to occur while reading the interrupt status. Note that disabling interrupts should be avoided if possible and that NMI (Non Maskable Interrupts) will still occur. If a HALT is executed when interrupts are disabled the system will stop. HALT should not be used anyway, nor should EI, DI or IM <n>.