Here's something I found regarding the Cathode Ray Tube Controller spec:
Vertical Timing
The Vertical Timing section consists of the Scan Line Counter, Character Row Counter, Registers R4 through
R9, the Vertical Control logic block, and associated Coincidence Circuits.
The Scan Line Counter counts from zero until coincidence with Register R9 synchronously resets the Scan Line
Counter and synchronously increments the Character Row Counter. The Scan Line Counter counts the Scan
Lines composing a character row, and the Character Row Counter counts the character rows comprising a
vertical frame.
The Character Row Counter coincidence with R4 and the residual Scan Line count represented by R5 marks the
end of a vertical frame.
The Character Row Counter coincidence with Register R6 marks the end of the active display portion of the
vertical frame measured in character rows.
The Character Row Counter coincidence with Register R7 marks the beginning of vertical retrace with Vertical
Sync (VS) going active high. VS remains high for a fixed period of 16 scan lines.
Register R8, Interlace Mode Register, effects the Vertical Timing according to its programming. Normal Sync
(Non-Interlace) mode displays the same field each frame. Interlace Sync Mode splits a frame into even and
odd fields. Vertical Sync (VS) active high is delayed one-half scan line at the end of even fields. For Interlace
Sync & Video Mode, in addition to the VS delay on even fields, the Row Address counter sequences on even
fields through 0,2,4,… counter values while on odd fields, through 1,3,5,… counter values.
Seems like your second guess would be right.
