Other undocumented information on strobes.
What has not been discussed previously is how Paula handles VERTB in INTENA/INTREQ (to manage BLIT it uses the INT3 PIN and COPER via 'software').
We known that Paula also uses STROBEs, but how? Now we know
Internally it maintains a
flag which is set to 0 by the STRHOR strobe and is set to 1 by both the STRVBL and STREQU strobes.
When there is
flag transition from 0->1 then VERTB is triggered.
This results in different behavior in each chipset combination, which can be summarized in the following table:
A1000 Agnus:
Line 0: STRHOR
Line 1: STREQU -> Paula VERTB, OCS Denise BLANK enable
OCS Agnus:
Line 0: STRVBL -> Paula VERTB, ECS Denise BLANK enable
Line 1: STREQU -> OCS Denise BLANK enable
ECS Agnus:
Line 0: STREQU -> Paula VERTB, OCS Denise BLANK enable, ECS Denise BLANK enable
Line 1: STREQU
OCS Denise BLANK enable -> immediately at strobe position
ECS Denise BLANK enable -> next internal or programmed HBSTRT position after strobe was detected
Note the OCS line 0 which is strangely STRVBL
All the various changes appear to have been made between later versions of the chipset to accommodate VERTB and VBLANK (finally in ECS they fixed everything).
