IBM RT Series Hardware Reference Manual page 331

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TNL SN20-9844 (March 1987) to 75X0235

Scan Line Register

I/O DATA FIELD

Bits 00-05 NOT USED

Bits 06-15 SCAN LINE (0-1023)

Figure 10. Scan Line Register (Address X'D9F808')

Note: This register is initialized to X'OOOO' with a POR or reset adapter command.

This register contains the next scan line pointer that will be serialized by the video output

circuits. The table below shows the scan line value in relation to the vertical cycle.

Scan Line Values

Relative Vertical Period

Active display

Vertical front porch

Vertical Sync

Vertical back porch

Writing the scan line register should be avoided as it causes the vertical cycle to glitch, which

may cause a vertical roll on the display.

Extended Monochrome Graphics Adapter 19

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