Interrupts

Default Interrupt Handler

The default interrupt handler is at address $0038 in ROM and is automatically called by the Z80 in Interrupt Mode 1.

It:

• updates the system variable frame counter;
• scans the keyboard and updates the keyboard state system variables.

The default interrupt handler preserves all registers and has no external requirements.

You can replace the interrupt handler by either:

• Setting the Z80 to interrupt mode 2 (IM 2) and configuring the address for a custom interrupt handler. Due to the restrictions on the Z80's interrupt specification, this requires placing the interrupt handler at an address where the high and low bytes are the same.
• Using Next Memory Management to page out the ROM at address $0038 and replace it with a RAM page with your own routine at offset $38.

Interrupt Timing

The standard Spectrum ULA raises an interrupt every video frame. This is very commonly used in games to synchronize graphics with the updates of the screen.

You can adjust the timing of video interrupts using Video Line Interrupt Control Register ($22). This will allow you to add an extra interrupt that occurs at a particular line address, or disable the standard ULA interrupt.

Using this register to "move" the standard interrupt to just after the final line of the visible screen can increase the time available for processing between frames.

Hardware IM2 Mode

Different from the regular IM2 mode which, from the programmer's perspective, matches the behavior of the original ZX Spectrum, a new "Hardware IM2 mode" exists in the core version 3.02.00 and above. It allows configuring different interrupt handlers for different interrupt sources.

Hardware IM2 mode is turned on with bit 0 of Next Register 0xC0 ( ($C0)) and has a full set of prioritized and chained interrupts, each with well-defined separate vectors. In this mode, when the IM2 occurs the lower 8 address bits are set as described in the reference documentation of the C0 next register: https://gitlab.com/SpectrumNext/ZX_Spectrum_Next_FPGA/-/blob/master/cores/zxnext/nextreg.txt?ref_type=heads

Interrupt requests are cached and held while a higher priority interrupt is being serviced, then triggered (potentially late) as soon as the higher priority interrupt is returned. All returns in this mode have to be done with reti to allow the hardware to track the priorities.

In standard mode and on regular Spectrums, if an interrupt is missed (the /INT line is released before the interrupt is accepted) then the interrupt is lost forever.