*Bits 7..2 are ignored.
*Bits 1..0 define the interlace mode.
[[File:CRTC Interlace modes.png]]
For a CRTC character line with n raster lines, R9 must be set to n-1 on all CRTCs, regardless of the interlace settings. The exception is for CRTCs 0/3/4 in IVM mode, where R9 must be set to n-2.
==== Interlace sync mode (ISM) ====
In this mode, the same information is painted in both fields to enhance readability. Reprogramming the CRTC is not necessary.
==== Interlace sync and video mode (IVM) ====
In this mode, alternating lines are displayed in the even and odd field to double the resolution.
On the even field, the CRTC displays the lines for which VLC is even. On the odd field, the CRTC displays the lines for which VLC is odd.
It is necessary to reprogram the CRTC as if we were building a frame of 624 lines.
CRTC 2 is the exception: R4, R5, R6, R7 do not need to be reprogrammed as it considers each character line to be a double character line.
==== Interlace adjustment line ====
On all CRTCs, in both interlace modes, an additional line (the 625th line) is added automatically by the CRTC at the end of the even field. This line is added after the lines of the vertical adjustment mode.
<br>