Changes
Reverted edits by [[Special:Contributions/Executioner|Executioner]] ([[User talk:Executioner|Talk]]); changed back to last version by [[User:89.50.201.2|89.50.201.2]]
A new 8 bit memory mapped register (PRI) has been added within the ASIC at address 6800h, which is cleared at power up. If zero, the normal raster interrupt mechanism functions as before. Otherwise, an interrupt occurs instead at the end of the scan line specified.. The PRI can be reprogrammed as required to produce multiple interrupts per frame. The raster interrupt occurs immediately on the leading edge of the combined logic of HSYNC active AND raster matched. The raster line is calculated as Vertical Character Count * 8 or Vertical Line Count. If the HSYNC position and duration programmed in the CRTC causes the HSYNC to still be active on the first horizontal character of a new scan line, the interrupt may occur twice for the programmed scan line (once at the start of the line, then again when the HSYNC starts at the end of the same line). See section 2.7 below for general information on interrupts.
===Soft scroll facility===
The SAR must be loaded by the CPU with a physical RAM address between 0000h and FFFEh. This means that the most significant two bits select which pages 0 to 3 of the DRAM is used, and the remaining bits are the address relative to the page start. The DMA process is not affected by the RAM or ROM mapping registers, and will always fetch data from RAM and not ROM. Note that the least significant bit of the address is ignored, and the instructions are always fetched from word boundaries.
The pause prescaler counts N+1 scan lines (where N is the value written by the CPU), giving a minimum tick of 64us, and a maximum of 16.384ms. When set nonzero by a pause instruction, the pause counter for a particular channel is decremented every tick until it reaches zero. Therefore, if the PPR is set to a value N and a PAUSE M instruction is executed, the total delay time between the instruction before the PAUSE and that following the PAUSE will be M * (N+1) * 64us. Pauses of between 64us and 67s may thus be generated. If a DMA channel is executing a pause when the SAR is changed, the pause counter will continue to decrement. If the DMA channel is disabled, the pause will stop decrementing for the period in which it is disabled, but it will not reset and when the DMA is again enabled and the command at the current SAR address will not be executed until the pause is complete. Changing the prescale value mid-pause will vary the length of the pause (eg. If the prescale is set to 0 and a PAUSE 10 is executed and has already decremented 5 times, changing the prescale to 1 will cause the pause to have a duration of 15 scan lines).
The ASIC arbitrates accesses to the parallel interface device between the "DMA" channels and the CPU, allowing only one to access it at a time. CPU accesses to the 8255 could be held off by means of wait states for up to 8 microseconds if the "DMA" channel is currently executing a LOAD instruction. After a LOAD is executed, the ASIC must put the PSG address register back as it was before. To achieve this the 8255 parallel peripheral interface and the 74LS145 decoder have been integrated into the ASIC.
The exact timing is based on 1us cycles as follows. After the leading edge from HSYN from the 6845 there is one dead cycle followed by an instruction fetch cycle for each channel which is active (i.e. enabled and not paused). The execute cycles then follow for each active channel. All instructions execute in one cycle, except that LOAD requires at least 8 cycles. An extra cycle is added to a LOAD if the CPU is accessing the 8255, or two extra cycles if the CPU access was itself a PSG register write.
The top 5 bits of ASIC can produce interrupts from four sources: the vector will be supplied from bit D7-D3 of a memory mapped raster interrupt vector register (IVR) at address 6805h in the ASIC, and the next two bits will determine the source of the interrupt. Bit D0 of the vector supplied is always zerothree sound generator "DMA" channels.
Thus interrupt service software in an environment where DMA interrupts are used must inspect these bits, giving highest priority to the raster interrupt, because this interrupt is always cleared automatically.
Failure to observe this requirement may result in raster interrupts being missed. DMA interrupts must be acknowledged by writing a "1" to the relevant DCSR bit.
===Enhanced ROM cartridge support===
0-7 0-7
High Bank: Logical page (DFxxh) Physical page 1
0-127 (not disc page) 1
The two ROM disable bits in the existing mode and ROM enable register disable the ROM as before, wherever it is mapped, as does the ROMDIS signal from the expansion bus.
The "write through" mechanism, whereby writes to an area which is currently mapped as ROM actually write to the underlying RAM, still functions, wherever the ROM is mapped. However the write through mechanism cannot be used to access the register page. Write through also does not operate to the RAM from the register page.
===Analogue paddle ports===
6802h 2 N W SSA Screen split secondary start address
6804h 1 Y W SSCR Soft scroll control register
6808h 1 R ADC0 Analogue input channel 0
6C07h 1 (unused)
6C08h 2 N W SAR2 "DMA" channel 2 address pointer
6C0Ah 1 N W PPR2 "DMA" channel 2 pause prescaler 6C0Bh 4 (unused) 6C0Fh 1 Y R/W DCSR "DMA" control/status register</pre>
