Posted By
MMS
on 2021-05-03
14:02:48
 |  Re: World of Commodore 2014 pictures, video, reports
I am far from any HW expert, but for me it seems the BA signal is just an output (an indicator) that you have full access to the data and addresses.
BTW it would have the same meaning as the DMA, just with opposite logic. Instead of pulling the DMA signal like on C64 (and wait 3 cycles after setting it), here the external cartridge may listen to the BA signal and keep the Z80 on hold till TED is not on the border area and not indicating it on the BA line. then you need to wait 3 cycles before writing.
The Z80 has a WAIT line, so the Z80 will suspend the operation (keep in T2 state), so it cannot collide with the TED on the same RAM area.
AEC may work in a similar waay as on C64.
As Jim Brain tols on a forum:
Jim Brain (br...@msen.com) wrote:
": If you REALLY want to read RAM from the internal C64 DRAM, you must disable
: the 6502 inside the 64 and use its cycles for your activities. Hmmmm,
: although I am nowhere near my schematic, you need to look at BE line, and
: as I recall AEC.
: I will go home and check my schematic, but you should be able to discern
: it from the schem. The VIC-II can disable the 6502 when it needs to
: "steal" cycles, so you need to behave as the 6502 does when reading/writing RAM.
: However, your circuit must make sure the 6502 never executes another
: instruction while your circuit is active, because the RAM in the 64 is
: never idle. "
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