💾 Archived View for mirrors.apple2.org.za › archive › ground.icaen.uiowa.edu › MiscInfo › Hardware ›… captured on 2024-07-09 at 03:29:16.
⬅️ Previous capture (2023-01-29)
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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
e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch