Skeetendo

’Cause all games were better on the GBC

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#1 2012-06-10 23:59:39

sub12
New member
Registered: 2011-03-02
Post 3/4

What is echo ram really for?

I've tried looking around and all I get is 'reserved space'. But there must have been some reason it's there, or we could have had twice as much RAM to use! Does anyone know why Nintendo actually put it in there?

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#2 2012-06-11 00:12:53

470/701

Re: What is echo ram really for?

It’s a bug in the hardware. Some of the address lines are tied to others, so reading from ECHO gives you data from other parts of memory.

They could have changed it, but that would have broken backwards compatibility (from what I’ve heard, some games did read from echo, even though Nintendo warned against it). Later, the GBC did get more memory without sacrificing backwards compatibility, by adding more WRAM banks.

#3 2012-06-11 00:33:23

sub12
New member
Registered: 2011-03-02
Post 4/4

Re: What is echo ram really for?

Ah, thanks man. I guess they just didn't want to admit to the error when they told everyone it was prohibited.

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#4 2012-06-11 22:10:17

Tauwasser
Member
Registered: 2010-10-16
Post 285/448

Re: What is echo ram really for?

It's not an error o.o" It's actually just what it is, an are that was not supposed to be used.
This is basically what happens when you do minimal address decoding with don't-care states. This just happens to be one of them and whoever made the map decided that one of the higher address bits was not needed for decoding. You kind of have a very childish approach to this.

7654|         Map to
-------------------------
0XXX| Cartridge (ROM)
100X| VRAM
101X| Cartridge (SRAM)
11XX| WRAM
1110| OAM
1111| CPU

It just so happens that WRAM addresses are decoded using address bits 4:0 only. Which is why E0 ^= C0 under this assumption.

cYa,

Tauwasser

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