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Notes on scroll registration

When performing scroll registration using the SGL library function “slScrAutoDisp”, the function may return a return value of -1 in some cases. This means that the function “slScrAutoDisp” failed to register the scroll.
When scroll registration fails, the cause is summarized in the following four.

● Cause of scroll registration failure

1) Limit scrolling by the number of colors in the normal scroll screen

An attempt was made to register a scroll surface that should not be used by setting the number of colors of NBG0 and NBG1.

2) Bank occupancy by rotating scroll data

The scroll data of the rotary scroll screen and the normal scroll screen are stored in the same VRAM bank.

3) Pattern name data storage bank restrictions

Pattern name data is stored in both VRAM banks having the same end number.

4) Deviation of allowable scroll function

The number of scroll surfaces you attempted to register exceeds the number of scroll functions and the number of faces that can be selected at the same time.

To solve this, take the following actions.

● How to deal with failed scroll registration

1) Corrective action

Decrease the number of colors of NBG0 or NBG1, or give up registration of NBG2 or NBG3.

2) Corrective action

The scroll data for the rotary scroll screen and the normal scroll screen are stored in separate VRAM banks.

3) Corrective action

Observe VRAM bank storage restrictions for pattern name data.

4) Corrective action

Reduce the number of scroll planes to be registered, reduce the number of characters in the character pattern during function setting, increase the value of the reduction setting, and avoid stuffing too much scroll screen scroll data into one VRAM bank Please try.

VRAM access restrictions

The cause of 4) is based on access restrictions on VRAM banks. However, it is very difficult to explain VRAM access restrictions and to read the accompanying contents. Therefore, here, the solution and countermeasures are taken by simply reducing the scroll function and the number of faces.
For details on VRAM access, refer to “HARDWARE MANUAL vol.2: VDP2 User's Manual”.

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