Re: 1541-II write protection switch

From: David Wood (
Date: 2002-02-01 10:22:18

Last DANG try! :((((((((((

I found this in my website, and am wondering about the likelyhood of it
Phase 2: DMA
We prepare address and place it, unmultiplexed on the bus
C64 sets up Row address, toggles RAS
c64 sets up Col address, asserts CAS for data read/write
we grab the byte
we change the low byte of the address to addr+1
we change the high byte of the address to a known non-ram area (I/O, kernal,
(above two happen at the same time)                             whatever)
We change the high byte back, which causes the PLA to re-assert CAS.
we read the next byte.

end of phase 2/dma

Neat idea, but there's just not enough time in 500ns to do it on a c64
clk->caslo is 229ns max
PLA propogatoin is 80ns max
ras precharge 80ns 
cas precharge 40ns

clk->cas		229ns
cas->casram		 80ns
casram->data		 50ns (150ns ram)
cas->casram		 80ns
casram->data		 50ns (150ns ram)
total 			489ns

The only limitation I can see is not being able to change rows in the same
clock phase.   

If this works, a c64 with fast enough ram (150ns FP ram, which is standard
in most) could do 4M/sec when the vicII's completely silent (screen off),
and 2M/sec during non-badline display.

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