In this mode, interrupts are prioritized in a fixed sequence. The raster interrupt has the highest priority, followed by DMA channels 2 down to 0 respectively.
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The register IVR (at address 6805h) supplies the top 5 bits of the vector. It is undefined at reset except that bit0 will be set to 1. Software should therefore always set up the IVR Therefore, before placing the CPU in vectored interrupt , always set up the IVR so that the top 5 bits are defined.
Bit0 of the IVR controls whether DMA channel interrupts are automatically cleared.