This Amstrad Plus ASIC performs many additional features that the old CPC series couldn't: the "Plus Features".
*12-bit colour palette*Hardware Sprites.sprites*Soft Vertical and horizontal per-pixel hardware soft scrolling (in complement with register 12&13 of the CRTC)
*Screen splitting
*Programmable and vectorised raster interrupts*Vectored interrupts
*DMA sound channels
*Specific ROM switching
*8-bit printer port (with bit3 of CRTC register 12)
*Analog joystick port
<br>
The 8-bit register SSCR (at address 6804h) controls soft scrolling by pixels rather than by characters. Setting this register to 0 (the default value at power-up) disables the soft scroll feature. The soft scrolling mechanism affects the entire main screen, regardless of the split screen feature, but does not affect sprites.
* Bit3Bits3..0 of SSCR defines define a horizontal delay between 0 and 15 high-resolution (mode 2) pixels, shifting the screen image to the right by the programmed value. This causes pixels to be lost behind the right border and random data to appear on the left. Also the programmer must ensure that the delay value is a multiple of the number of bits per pixel.* Bit6Bits6..4 of SSCR is added are summed to the least significant 3 bits of the scan line address, determining which of the eight 2k blocks contains the data for the first scan line on the screen. This shifts the display up by the programmed number of scan lines, causing the first lines to be lost and extra lines to appear at the bottom.
* Bit7 of SSCR, when set, extends the border to cover the first 2 bytes (16 high-resolution pixels) of each scan line, masking bad data caused by the horizontal soft scroll. When using horizontal soft scroll, always set this bit to maintain consistent screen width.
== Programmable Raster Interrupt ==
The 8-bit memory-mapped register PRI (at address 6800h) specifies the scan line where the interrupt occurs. The interrupt will occur at the end of that scan line. Setting this register to 0 (the default value at power-up) reverts to the classic R52 raster interrupt system of the [[Gate Array]] R52 raster interrupt system instead.
PRI can be reprogrammed as required to produce multiple interrupts per frame.
Additionally, on Amstrad Plus, we have multiple sources of interrupts as each DMA sound channel can trigger an interrupt.
<br>
== Vectored Interrupt ==
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.
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 before placing the CPU in vectored interrupt so that the top 5 bits are defined.
Bit0 of the IVR controls whether DMA channel interrupts are automatically cleared.
<br>
A The DMA control and status register (DCSR(at address 6C0Fh) controls which channels are currently enabled, and also tells the CPU which channel is interrupting:* The Bits2..0 are the channel enable bits in this register enable each . When set to "DMA1" it enables the corresponding DMA channel separately, and . It can be set by the CPU, and cleared by either the CPU, a STOP instruction, or power on rest.* The Bits7..4 are the interrupt bits are . An interrupt bit is set to "0" when a the corresponding channel is requesting an interrupt, and cleared when the CPU writes a "1" to the appropriate bit.* The INT signal of the ASIC is the compositing of all the interrupt bits of DCSR by using the AND function. INT is active at "0" if at least one of the interrupt bits is "0". <br> == Vectored Interrupt == The ASIC always provides an interrupt vector on interrupt request. The register IVR (at address 6805h) supplies the top 5 bits of the vector provided to the CPU. IVR is undefined at reset except that bit0 will be set to 1. Therefore, before placing the CPU in vectored interrupt mode, always set up the IVR so that the top 5 bits are defined. Bits2..1 of the IVR register are unused. Bit0 of the IVR register controls whether DMA channel interrupts are automatically cleared. Interrupts are prioritized in a fixed sequence. The raster interrupt has the highest priority, followed by DMA channels 2 down to 0 respectively. ==== Interrupt Vector used on Z80 IM2 mode ===={| class="wikitable"|-!Interrupt Vector!Signal source!Value|-|A15..A8||Z80||Register I|-|A7..A3||ASIC||IVR register bits7..3|-|A2..A1||ASIC||00 = DMA chan 2, 01 = DMA chan 1, 10 = DMA chan 0, 11 = DMA raster|-|A0||ASIC||Always 0|}
<br>