Interrupts

Two system calls are available to enable and disable interrupts. It is very important to use these calls 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>.

OZ_DI, disable 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

OZ_EI, restore old interrupt state

CALL $0054

IN:

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

OUT:        -

Registers changed after return:
            AFBCDEHL/IXIY same
            ......../.... different

Thus one might do:

            call OZ_DI
            push af

            ; operations requiring disabled interrupts

            pop  af
            call OZ_EI

and be assured for the interrupt status is preserved.