Interrupts

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IME - Interrupt Master Enable Flag (Write Only)

 0 - Disable all Interrupts
 1 - Enable all Interrupts that are enabled in IE Register (FFFF)

The IME flag is used to disable all interrupts, overriding any enabled bits in the IE Register. It isn't possible to access the IME flag by using a I/O address, instead IME is accessed directly from the CPU, by the following opcodes/operations:

 EI     ;Enable Interrupts  (ie. IME=1)
 DI     ;Disable Interrupts (ie. IME=0)
 RETI   ;Enable Ints & Return (same as the opcode combination EI, RET)
 <INT>  ;Disable Ints & Call to Interrupt Vector

Whereas <INT> means the operation which is automatically executed by the CPU when it executes an interrupt.

The effect of EI is delayed by one instruction. This means that EI followed immediately by DI does not allow interrupts between the EI and the DI.

FFFF - IE - Interrupt Enable (R/W)

 Bit 0: V-Blank  Interrupt Enable  (INT 40h)  (1=Enable)
 Bit 1: LCD STAT Interrupt Enable  (INT 48h)  (1=Enable)
 Bit 2: Timer    Interrupt Enable  (INT 50h)  (1=Enable)
 Bit 3: Serial   Interrupt Enable  (INT 58h)  (1=Enable)
 Bit 4: Joypad   Interrupt Enable  (INT 60h)  (1=Enable)

FF0F - IF - Interrupt Flag (R/W)

 Bit 0: V-Blank  Interrupt Request (INT 40h)  (1=Request)
 Bit 1: LCD STAT Interrupt Request (INT 48h)  (1=Request)
 Bit 2: Timer    Interrupt Request (INT 50h)  (1=Request)
 Bit 3: Serial   Interrupt Request (INT 58h)  (1=Request)
 Bit 4: Joypad   Interrupt Request (INT 60h)  (1=Request)

When an interrupt signal changes from low to high, then the corresponding bit in the IF register becomes set. For example, Bit 0 becomes set when the LCD controller enters into the V-Blank period.

Interrupt Requests

Any set bits in the IF register are only <requesting> an interrupt to be executed. The actual <execution> happens only if both the IME flag, and the corresponding bit in the IE register are set, otherwise the interrupt 'waits' until both IME and IE allow its execution.

Interrupt Execution

When an interrupt gets executed, the corresponding bit in the IF register becomes automatically reset by the CPU, and the IME flag becomes cleared (disabeling any further interrupts until the program re-enables the interrupts, typically by using the RETI instruction), and the corresponding Interrupt Vector (that are the addresses in range 0040h-0060h, as shown in IE and IF register decriptions above) becomes called.

Manually Requesting/Discarding Interrupts

As the CPU automatically sets and cleares the bits in the IF register it is usually not required to write to the IF register. However, the user may still do that in order to manually request (or discard) interrupts. As for real interrupts, a manually requested interrupt isn't executed unless/until IME and IE allow its execution.

Interrupt Priorities

In the following three situations it might happen that more than 1 bit in the IF register are set, requesting more than one interrupt at once:

 1) More than one interrupt signal changed from Low
    to High at the same time.
 2) Several interrupts have been requested during a
    time in which IME/IE didn't allow these interrupts
    to be executed directly.
 3) The user has written a value with several "1" bits
    (for example 1Fh) to the IF register.

Provided that IME and IE allow the execution of more than one of the requested interrupts, then the interrupt with the highest priority becomes executed first. The priorities are ordered as the bits in the IE and IF registers, Bit 0 (V-Blank) having the highest priority, and Bit 4 (Joypad) having the lowest priority.

Nested Interrupts

The CPU automatically disables all other interrupts by setting IME=0 when it executes an interrupt. Usually IME remains zero until the interrupt procedure returns (and sets IME=1 by the RETI instruction). However, if you want any other interrupts of lower or higher (or same) priority to be allowed to be executed from inside of the interrupt procedure, then you can place an EI instruction into the interrupt procedure.

Interrupt Service Routine

According to Z80 datasheets, the following occurs when control is being transferred to an interrupt handler:

1. Two wait states are executed (2 machine cycles pass while nothing occurs, presumably the CPU is executing NOPs during this time).
2. The current PC is pushed onto the stack, this process consumes 2 more machine cycles.
3. The high byte of the PC is set to 0, the low byte is set to the address of the handler ($40,$48,$50,$58,$60). This consumes one last machine cycle.

The entire ISR should consume a total of 5 machine cycles. This has yet to be tested, but is what the Z80 datasheet implies.