== CRTC register differences ==
It is not possible to read from all the internal registers, this table shows the read/write status of each register for each type: {| class="wikitable"!rowspan=2|Register Index!rowspan=2|Register Name!colspan=3|Type|-!0!1 & 2!3 & 4|-|0||Horizontal Total (-1)||colspan=3 style="text-align: center;"|Write Only|-|1||Horizontal Displayed||colspan=3 style="text-align: center;"|Write Only|-|2||Horizontal Sync Position||colspan=3 style="text-align: center;"|Write Only|-|3||Horizontal and Vertical Sync Widths||colspan=3 style="text-align: center;"|Write Only|-|4||Vertical Total (-1)||colspan=3 style="text-align: center;"|Write Only|-|5||Vertical Total Adjust||colspan=3 style="text-align: center;"|Write Only|-|6||Vertical Displayed||colspan=3 style="text-align: center;"|Write Only|-|7||Vertical Sync Position||colspan=3 style="text-align: center;"|Write Only|-|8||Interlace and Skew||colspan=3 style="text-align: center;"|Write Only|-|9||Maximum Raster Address (aka Number of Scan Lines) (-1)||colspan=3 style="text-align: center;"|Write Only|-|10||Cursor Start Raster||Write Only||Write Only||Read/Write|-|11||Cursor End Raster||Write Only||Write Only||Read/Write|-|12||Display Start Address (High)||Read/Write||Write Only||Read/Write |-|13||Display Start Address (Low)||Read/Write||Write Only||Read/Write|-|14||Cursor Address (High)||colspan=3 style="text-align: center;"|Read/Write|-|15||Cursor Address (Low)||colspan=3 style="text-align: center;"|Read/Write|-|16||Light Pen Address (High)||colspan=3 style="text-align: center;"|Read Only|-|17||Light Pen Address (Low)||colspan=3 style="text-align: center;"|Read Only|-|} '''Notes''' * On CRTCs 0/1/2, if a Write Only register is read from, "0" is returned. The register accessing scheme on CRTCs 3/4 makes it impossible to happen. * CRTC types 3 and 4 are identical in every way, except for the unlocking mechanism, split screen, hardware soft scroll and 8-bit printer port functionalities specific to the ASIC. * See the document "Extra CPC Plus Hardware Information" for more details.
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