CPCWiki forum
General Category => Amstrad CPC hardware => Topic started by: AxelStone on 13:42, 14 March 18
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Most probably this has been already discussed, anyway here goes. Is there any hardware restriction that makes Mode 4 only accessible with cartridges? It's a great lost for CPC+. For games in cassette / FDD, what extra features provides CPC+ against normal CPC?
Thanks.
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Mode 4 ?
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Extended mode of CPC+. I've been answered in other chat, it seems that it can be easily used in other formats simply using a command (CALL MODE,4 or something similar). Games like PRehistorik 2 uses it.
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They fooled you. The CPC as the 6128plus only have modes 0, 1, 2 and 3 (the latter one is not too much useful). If people talk about other modes they may refer to their software which probably just define some kind of other mode.
But to come to your question. Since there is not a CPC+, but a 464plus, 6128plus and GX4000: They all need to be unlocked by sending a code to the CRTC's I/O address. After that all new 'plus' features can be used freely in any kind of software, the medium (cartridge, tape, disc) does not have any influence.
The only advantage of using cartridges is advanced ROM banking (and no loading time). But that's about it. :)
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Advanced ROM banking (and no loading time). But that's about it. :)
Conventionnal ROM does have advanced ROM banking (and you can have up to 2M of conventionnal ROM instead of cartridge limited to 512K)
But cartridge is convenient!
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On a CPC: ROM can only be banked in at &C000 (exceptions: Lower ROM, Pseudo-ROMs of the SF2)
Max ROM: 4 MB
On a Plus: Enhanced ROM Banking.
Max ROM: 2 MB +Cartridge ROMs = 2.5 MB
What was forgotten?
For the Plus See:
In addition, new bits are defined in the mode and ROM enable (MRER) register at I/O address 7Fxxh, Previously D7 = 1 and D6 = 0 to select this register, and D5 should be 0.
This has been modified such that, if this register is written with D5 = 1, the bottom five bits are redefined. This new register is known as the secondary ROM mapping register (RMR2). D4 and D3 control the address of the low bank, and also whether the memory mapped registers are enabled at 4000 to 7FFFh.
D4 D3
0 0 Low bank ROM = 0000 to 3FFFh, register page off - &7FA0
0 1 Low bank ROM = 4000 to 7FFFh, register page off
1 0 Low bank ROM = 8000 to BFFFh, register page off
1 1 Low bank ROM = 0000 to 3FFFh, register page on - &7FB8!
D2 to D0 determine which of the lower 8 pages of the cartridge ROM appear at the low bank address.
[size=0pt]The default is page 0.[/size]
[size=0pt][/size]
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Enhanced banking of the + is not specific to the cartridge. That's precisely why the M4Wifi can emulate the cartridge on a regular extension slot. Or i miss something?
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The secondary ROM mapping is a banking feature provided by the ASIC, not cartridges per se. If the M4 "knows" about it and provides the right response in response to the right commands (which basically requires tracking the ASIC enabled status and allowing a slightly different interpretation of certain gate array commands) then it is perfectly possible to mimic the behaviour - in fact you could mimic it on a standard CPC too if you really wanted too. I don't know if the M4 does handle RMR2 usage though, it's not widely used by existing cartridge software.
The whole concept that some Plus feature are somehow limited to cartridges is a ruse put out by Amstrad who were probably hoping that it would be treated that way (which fell by the wayside after the GX basically died). I suppose it's not, strictly speaking, a lie since everything running on a Plus is to some extent "cartridge" software - after all you need at least a BASIC cartridge installed to run anything....
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The secondary ROM mapping is a banking feature provided by the ASIC, not cartridges per se. If the M4 "knows" about it and provides the right response in response to the right commands (which basically requires tracking the ASIC enabled status and allowing a slightly different interpretation of certain gate array commands) then it is perfectly possible to mimic the behaviour - in fact you could mimic it on a standard CPC too if you really wanted too. I don't know if the M4 does handle RMR2 usage though, it's not widely used by existing cartridge software.
M4 does watch the gate array for RMR and RMR2 and map accordingly and yes it works to some extend on regular CPC too, ie. you can play the Barbarian "plus" cartridge on a regular CPC with M4.
Of course it's not easily possible to map roms to 0x4000 and 0x8000 on a regular CPC.
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Enhanced banking of the + is not specific to the cartridge. That's precisely why the M4Wifi can emulate the cartridge on a regular extension slot. Or i miss something?
Could be that the M4 is simulating the Cartridge. :) I mean plugging ROM into the exp. port or the cart. port. Singnals for reading ROMs are similar. :)
EDIT: Everything told in previous posts :)