Apple SoftCard CP/M memory map (was: Re: Problems transferring DOS3.3 from PC to IIe :( )

[pausch@saafNOSPAM.se (Paul Schlyter)]

In article <processoroverclocked-1705002203220001@nn-157-22-180-2.napanet.net>,
Jon Bettencourt <processoroverclocked@33mhz.com> wrote:
 
>> Also, could someone enlighten me to what card would be in slot 4 with a
>> Z80A CPU - the IIe works fine without the card in, and the LED doesnt
>> come on on the card.
>
> That's a CP/M card. The LED comes on when you start using it (i.e. boot
> up with a CP/M disk).
 
If the 6502 *writes* to the address where normally the boot ROM code
starts ($C400 for a CP/M card in slot 4), then the 6502 is switched
off and the Z80 starts executing from Z80 address 0000h.  Before
doing that though, one had better load some Z80 code at that address,
so the Z80 does something sensible.
 
To switch back to 6502 mode, the Z80 must write to the same address
(E400h) -- then the Z80 stops and the 6502 resumes executing.
 
Since the 6502 is a dynamic CPU, it cannot remain idle for more than
about 50 microseconds, or else it'll "forget" the contents of its
internal regusters, and then you'd be in real trouble.  To avoid this,
the Z80 ticks at 4 Mhz for 2 cycles, then remains idle for two cycles
during which the 6502 is refreshed (the 6502 is allowed to fetch one
byte, but it doesn't use that byte for anything).
 
The Z80 card contains address translation logic, to avoid having
the Z80 address 0000h coincide with the 6502 address $0000.  The
translation works as follows:
 
             Z80 address         6502 address
 
PSP          0000h - 0FFFh       $1000 - $1FFF
TPA start    1000h - 1FFFh       $2000 - $2FFF
             2000h - 2FFFh       $3000 - $3FFF
             3000h - 3FFFh       $4000 - $4FFF
             4000h - 4FFFh       $5000 - $5FFF
             5000h - 5FFFh       $6000 - $6FFF
             6000h - 6FFFh       $7000 - $7FFF
             7000h - 7FFFh       $8000 - $8FFF
             8000h - 8FFFh       $9000 - $9FFF
             9000h - 9FFFh       $A000 - $AFFF
             A000h - AFFFh       $B000 - $BFFF       End of motherboard RAM
             B000h - BFFFh       $D000 - $DFFF       Start of Language Card RAM
TPA end      C000h - CFFFh       $E000 - $EFFF
BDOS/BIOS    D000h - DFFFh       $F000 - $FFFF       End of Language Card RAM
I/O area     E000h - EFFFh       $C000 - $CFFF       I/O area        
             F000h - F0FFh       $0000 - $00FF       6502 page zero
             F100h - F1FFh       $0100 - $01FF       6502 stack
I/O drivers  F200h - F3FFh       $0200 - $03FF
Text screen  F400h - F7FFh       $0400 - $07FF
             F800h - FFFFh       $0800 - $0FFF       CP/M RWTS (6502 code)
 
This assumes a Language Card: if a Language Card is present, CP/M
enables it and disables the onboard ROM's.  If the Language Card is
absent, the TPA is 12K smaller.  Note how the Z80 ==> 6502 address
translation "hides" the "hole" in the memory map due to the Apple
II I/O area at $C000-$CFFF, and makes motherboard and Language Card
RAM appear as one contiguous RAM area.
 
-- 
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Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40,  S-114 38 Stockholm,  SWEDEN
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