Changes
/* CSYNC signal */
== Introduction ==
The gate array Gate Array is a specially designed chip exclusively for use in the Amstrad CPC and was designed by Amstrad plc.
In the CPC+ system, the functions of the Gate-Array are integrated into a single [[ASIC|ASIC]]. When the ASIC is "locked", the extra features are not available and the ASIC operates the same as the Gate-Array in the CPC allowing programs written for the CPC to work on the Plus without modification. The ASIC must be "un-locked" to access the new features.
In the [[KC Compact]] system, the functions of the Gate-Array are "emulated" in TTL chips, [[CIO Overview|CIO]], and its color translation EPROM.
In the "cost-down" version of the CPC6128, the functions of the Gate-Array are integrated into a ASIC.
The Gate Array is described here is the one found in a standard CPC.
== CSYNC signal ==
On CPC, the HSYNC and VSYNC signals are received from the [[CRTC]]. These signals are then modified and merged by the Gate Array as C-HSYNC and C-VSYNC and then merged into a single CSYNC signal that will be sent to the display.
When CRTC HSYNC is active, the Gate Array immediately outputs the palette colour black. If the HSYNC is set to 14 characters then black will be output for 14µs.
C-HSYNC and C-VSYNC are composited using the XNOR function. The resulting CSYNC signal produced by the Gate Array is 1 when inactive and 0 when active.
On a CPC monitor, the CSYNC is rendered in "absolute black". It is darker than the palette colour black output by the Gate Array. The electron beam is basically turned off. Turning up the brightness level won't make it any brighter.
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This function is not available in the Gate-Array, but is performed by a device at the same I/O port address location. In the CPC464, CPC664 and KC compact, this function is performed in a memory-expansion (e.g. Dk'Tronics 64K RAM Expansion), if this expansion is not present then the function is not available. In the CPC6128, this function is performed by a [[PAL16L8|PAL]] located on the main PCB, or a memory-expansion. In the 464+ and 6128+ this function is performed by the ASIC or a memory expansion. Please read the document on RAM management for more information.
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== Register 0 - Palette Index (Pen selection) ==
{|{{Prettytable|width: 700px; font-size: 2em;}}
|rowspan=2|'''Graphics Mode'''||colspan=8 style="text-align: center;"|'''VRAM byte'''||colspan=8 style="text-align: center;"|'''Displayed Pixels'''||rowspan=2|'''Definition'''
|-
|'''7'''
*[[Media:40010-simplified V03.pdf]] [https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/gate-array-decapped!/msg170713/#msg170713 Forum thread] Gate Array schematics - reverse engineered by Gerald
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== External links ==
