Floppy controller questions

[Dwight K. Elvey]

>From: "Allison" <ajp166 at bellatlantic.net>
---snip---
>
>ugly 8080 flavor for transfers up to 1KB:
>
>  ;  HL is memory pointer
>  ;  A is temp
>  ;  B is transfer length *4
>  ;  Zero flag affected by DCR B
>  DMAIO:  IN DMAport  ; wait if DMA is not asserted
>          mov M,A
>          INX H       ; HL<-HL+1
>          IN DMAport  ; wait if DMA is not asserted
>          mov M,A
>          INX H       ; HL<-HL+1
>          IN DMAport  ; wait if DMA is not asserted
>          mov M,A
>          INX H       ; HL<-HL+1
>          IN DMAport  ; wait if DMA is not asserted
>          mov M,A
>          INX H       ; HL<-HL+1
>          DCR  B      ; B is dmatransfer down counter
>          JNZ  DMAIO  ; loop if not Zero, more to go.
>          ...
>

Hi
 As I recall, you could move the DCR B before the last
IN DMAport. As I recall, INX H didn't effect the zero
flag ( or was that the carry flag ? ). In any case,
This kept the last time shorter.
Dwight

>It's 35 cycles in z80 for the slowest part of the loop
>and 34 for 8080.  Z80 IN port is 11cy!
>
>Why four bytes pwer transfer? one it makes doing 512 or
>1024 bytes per sector with a 8bit counter (lower overhead)
>and doing a 765 format only requires 4 bytes per sector
>(C, H, R and N)!
>
>A 2mhz 8080 will actually run this for 8" DD (13us case)
>successfuly even though the last leg of the loop is 
>17.5us as you have 16us +13us worst case before an 
>overrun occurs and the next step in the loop is not 
>as slow. Note waits for refresh will blow the works
>and Z80s systems that use processor refresh are not 
>likely to behave well if the FDC hangs (no disk or 
>blank inserted) too long.
>
>A 6502 can easily do it.
>
>>From a system perspective I prefer DMA.  I like to free 
>up CPU cycles to do "stuff" and make hardware do repeatitive
>stuff like basic transfers.  However the DMA does not have \
>to be complex or even a LSI (8257 or 8237). A simple gating
>logic plus an upcounter of sufficient length is enough and 
>it can even be to a fixed address (host buffer in the case 
>of CP/M deblocking). Things like background tasks are then
>easier to implement to utilize otherwise wasted IO loops 
>(wait for keyboard!). The most obvious interrupt driven 
>background tasks to implement are printer output buffering
>or modem input buffering.
>
>
>Allison
>
>
>
>