plus3: update README ready for doing some actual work on this port

diff --git a/Kernel/platform-plus3/README.md b/Kernel/platform-plus3/README.md
index 487fb62..6e211ba 100644 (file)
--- a/Kernel/platform-plus3/README.md
+++ b/Kernel/platform-plus3/README.md
@@ -1,4 +1,4 @@
-`An FUZIX target for ZX Spectrum +2A/+3`
+`A FUZIX target for ZX Spectrum +2A/+3`
 
 The +2A and +3 have the following choice of memory configurations on top of
 the standard 128K spectrum
@@ -11,18 +11,30 @@ conf2       [4] [5] [6] [3]
 conf3  [4] [7] [6] [3]
 ```
 
+We have an interesting limitation: unlike the 128K spectrum banks 4-7 are
+all contended (slower). In addition pointing I into those banks will crash
+the machine. Providing we keep RAM in the low 16K we can avoid IM2 however.
+
 That gives us a conventional low 0/1 and 4/5 for user space with the kernel
-using 2/3/6/7.
+using 3/6/7. Given the memory napping limits it actually probably makes
+more sense to eventually allow one 48K and one 32K mapping so most processes
+can run two at a time but one big one is allowed. That implies we get our
+chmem act together.
 
-Kernel maps are then
+We then map
 
-3 = common (always mapped high)
-2/6 = banked (at 0x8000)
-7 = banked with screen (at 0x4000)
+0/1/2/3        -       user process 0: 48K, 16K common
+4/5/6/3 -      user process 1: 32K, 16K common, 16K is kernel reserved
+4/7/6/3 -      kernel: 48K minus screen, common 16K (some kernel in it)
 
-although 7 does not appear to be part of the banks we cannot map 2 and 7
-together so it's effectively banked.
+As an aside this only works for the +2A/+3. The various clones even with
+lots of RAM clone the original 128K and add more pages of memory in the top
+16K bank. Some allow the low 16K to become RAM (eg for CP/M) but have
+4000-BFFF fixed.
 
-Alternatively we could go with a single 64K swapping user space with
-kernel mapped normally at 4-7  (with screen hole at C000) and user at 0-3
-and a small copied common in bank 3 and 7
+There are also various add in boards some of which can work on the +2A/+3
+that provide alternative RAM and ROM banks for the low 16K. We don't make
+use of them in this configuration althogh in future it would make sense to
+make the kernel mapping in such a case map DivMMC or similar RAM into the
+low 16K when in kernel mapping so the kernel becomes mmc/7/3 and we leave 6
+free to allow a pair of 48K processes.