Retrosoftware — Smooth Vertical Scrolling

Retrosoftware — “How to do the smooth vertical scrolling”

Tutorial by Richard Talbot-Watkins, hosted at retrosoftware.co.uk (2008). Practical introduction to two related raster techniques:

1. Vertical rupture — split a single TV frame into two or more CRTC cycles, each with its own screen-start address. Foundation for split-screen and status panels.
2. Smooth vertical scrolling — abuse R5 (vertical total adjust) across two CRTC cycles to add 0-7 extra scanlines to the top border, shifting the displayed screen by sub-character-row amounts while keeping the PAL total at 312 lines.

Accompanying source:

• raw/code/vrupt.6502 — BeebASM source for the rupture demo (annotated rework by jbnbeeb, 2015, of Talbot-Watkins’s original BBC-ASM listing).
• raw/code/smoothscroll.bas — BBC BASIC + inline assembler source for the smooth-scroll demo (Talbot-Watkins’s original).

Key technical claims

• PAL frame = 39 character rows. Default MODE 2 has R7 = 34 (vertical sync at row 34), so 5 character rows remain after VSync before the cycle ends.
• Total scanlines formula: (R4+1) * (R9+1) + R5 = 39 * 8 + 0 = 312. Any change to R5 must be compensated for in another cycle’s R5 to keep the total at 312.
• R4, R6, R7 are read each cycle (not pre-latched). Reprogramming them mid-frame, during a CRTC cycle, affects the next read of those registers — i.e. immediately for R4 (cycle length), and for the field’s R7 (VSync position).
• R12/R13 are latched at the start of each CRTC cycle. Writing them mid-cycle changes the address used by the next cycle, not the current one. This is the lever rupture relies on.
• R5 (vertical total adjust) is added to the cycle’s row count as a number of extra scanlines after VSync — practically, extra top-border scanlines from the viewer’s perspective.
• VSync interrupt fires after the VSync pulse-width period (default 2 scanlines), not at the exact VSync edge. Timer setups for screen-on must account for this offset.
• R7 mid-frame rewrite “works” in this technique despite the Hitachi datasheet classifying it as NG. The trick is that rewrites happen during the previous CRTC cycle and settle before the new cycle samples them. NG applies to rewrites during the cycle whose VSync would be affected. See note in vertical-rupture.
• Rupture worked example geometry (MODE 2):
  • Top window: 16 rows, hardware-scrolled, screen address in range &5800-&7FFF (uses hardware wraparound at &8000 — requires 10K screen latched via &FE40 bit 4/5).
  • Bottom window: 16 rows, stationary, start &3000.
  • Cycle 1 (top): R4=15 (16 rows), R6=16, R7=255 (no VSync this cycle).
  • Cycle 2 (bottom): R4=22 (23 rows), R6=16, R7=18 (VSync at 23-5=18 to match normal MODE 2 position).
• Smooth-scroll geometry (24 displayed rows, MODE):
  • Cycle with VSync: R5 = 8 - line, R4 = 39 - 24 - 1 - 1 = 13 (rows count includes one extra row to absorb the two R5 fractions).
  • Cycle with playing area: R5 = line, R4 = 24 + 1 - 1 = 24 (so 25 rows of which 24 displayed + ‘line’ extra scanlines).
  • Screen disabled via Video ULA after VSync; re-enabled by System VIA T2 timer set at VSync IRQ, giving rock-steady visible top edge.
  • The timer load value compensates for R5 so the screen-on point is always at the same physical scanline.
• Why the lower status window matters: with two CRTC cycles, the bottom cycle naturally provides a visible “status” area. This gives the timer interrupt some slack — it can fire anywhere within the status row’s display and you’ll still hit the cycle-switch correctly. Without it, R6=0 with imperfect timing would let the CRTC briefly send display data to the TV.

Filed into

• vertical-rupture — new entity page; foundational technique.
• smooth-vertical-scroll — new entity page; builds on rupture.
• crtc-6845 — augmented practical-rewrite note: R4/R6/R7 mid-frame rewrites are safe when done during the cycle preceding the one they govern (the technique’s whole basis).
• hardware-scrolling — cross-link added to smooth-scroll page.
• index / log — standard updates.

Notes from the smoothscroll.bas listing

Initial CRTC register table (line 310 crtcvals):

R0	R1	R2	R3	R4	R5	R6	R7	R8	R9	R10	R11
127	80	98	&28	38	0	26	31	&F0	7	32	8

Note divergence from default MODE 2: R6=26 (not 32 — gives total displayed of 26 rows split as 25 scroll + 1 status), R7=31 (not 34 — VSync shifted), R8=&F0 (display delay=3, cursor delay=3, non-interlaced, no skew). Addressable latch (&FE40) is set to select the 20K screen (&3000-&7FFF) with LDX #4 : STX &FE40 : INX : STX &FE40 (clearing bit 4 then setting bit 5 — see system-via for latch wraparound bits).

Variables (zp): addr (R12/R13 word), line (sub-row 0-7), iline (latched copy used in the timer ISR), vsync/vsync2 (frame-count flags), irqcount (which IRQ next), delay (timer compensator).

Open follow-ups

• “Vertical rupture” terminology originates in the Amstrad CPC scene per the article — would be worth a brief etymology note if we ever add a cross-platform-techniques page.

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This wiki is curated by Claude following the LLM-Wiki methodology — a human curates source documents, the LLM compiles structured cross-linked markdown. Content may contain errors, omissions, or stale claims. For authoritative information refer to the original source documents in the bbc-documents GitHub archive.