From: Alan Cox <alan@etchedpixels.co.uk>
Date: Fri, 7 Nov 2014 00:50:09 +0000 (+0000)
Subject: README: Random ramblings about how a port might work
X-Git-Url: https://git.ndcode.org/public/gitweb.cgi?a=commitdiff_plain;h=0ad93881045662e0e26d18d55adb141c63862ca9;p=FUZIX.git

README: Random ramblings about how a port might work
---

diff --git a/Kernel/platform-msx1/README b/Kernel/platform-msx1/README
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index 00000000..0f5233fe
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+First wild guess at an MSX1 target
+
+And it is exactly that a guess 8)
+
+
+No memory paging routines implemented yet so not very useful!
+
+
+This will also almost certainly need udata copy on process switching at
+least until someone fixes sdcc to generate bankable code. At that point we
+may be able to bank the OS kernel in a cartridge.
+
+
+Q: should we work to the following map
+
+
+	0x0000-0x7FFF	User space / switchable with kernel
+			(in theory if kernel code is cartridge
+			 could we even use RAM + megaram?)
+	0x8000-0xBFFF	Kernel (end of code, data, highcode, common)
+	0xC000-0xFFFF
+
+or do we bite the bullet about relocatable binaries (which we need to do
+for the sinclair at least) and do
+
+	0x0000-0x00FF	Vectors
+	0x0100-????
+	0x4000-0xBFFF	User space/switchable with kernel rest
+	0xC000-0xFFFF
+
+which would allow the use of more typs of megaram without LDIR tricks.
+
+There are some other interesting tradeoffs on size handling if you make the
+OS a cartridge as well as you can then use main memory via LDIR and lazy swap
+the other 16K of app space for a 48K app so you only end up copying for
+switches between big apps but get 48K apps. That might mean binaries at
+0x4000-0xFFFF or 0x0000-0xBFFF, but either way means shunting the kernel
+around in the pure RAM case to avoid the overlap (eg loading the kernel
+at 0x4000 (with discard and bootstrap at 0x100))
+
+
+Other mappers that might be found include
+
+	- MSX2 mapper (in which case we should probably make the msx2
+		platform detect the early type VDP and use that ?)
+
+	- Zemina (requires LDIR copying stuff)
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