41CL: Second set of Extended Memory

[Ángel Martin]

You probably know already that the 41CL has 2MB of flash and 512kB of RAM capacity. Perhaps the most intriguing aspect of that additional RAM memory would be to make it somehow available to the OS; exceeding the built-in limits of the original design...

No doubt that would require patching the OS extensively, probably too much for a reasonable undertaking and a lot of effort to say the least.

But a lower-hanging fruit could be extending the Extended Memory capacity, definitely more tractable than the main memory space. With that idea in mind I've been fumbling with the X-Mem control and linking registers, trying to add additional EM-blocks to the "chain"; and exploring alternate EM configurations in RAM locations above 0x400.

So far the obvious tricks haven't been fruitfull, it appears there too patching the OS code would be required to get it to work. But using a bit of lateral thinking it's easy to come up with easy ways to back up all your X-memory, or even to have an alternative (dual) set of X-Mem configured, so you can switch back and forth between them.

Below is the quick and dirty FOCAL program to hot-swap the standard X-Memory configuration with another one located in the higher memory locations, above 0x400. It keeps the same structure (one base block of 128 registers plus two additional ones of 239 each), at equivalent addresses to maintain the parity. This maintains the compatibility for main-RAM back-ups as well, assuming of course that the switching routine would be running from ROM.

Interestingly it only uses PEEKR and POKER as non-standard functions, available in the CCD Module and a few of its derivatives.

Take it for a spin and report issues or ideas for enhancement, let's see where can this be taken to...

Code:



1    LBL "XMSWAP"    
2    "BASE"    provide feedback
3    AVIEW    
4    128    number of base registers
5    STO M    store counter in M
6    64    origin
7    XEQ 00    swap block
8    "EM-1"    provide feedback
9    AVIEW    
10    239    number of base registers
11    STO M    store counter in M
12    513    origin
13    XEQ 00    swap block
14    "EM-2"    provide feedback
15    AVIEW    
16    239    number of base registers
17    STO M    store counter in M
18    769    origin
19    LBL 00    swap block
20    CF 00    
21    RCL X    origin
22    1024    offset address
23    +    destination
24    LBL 01    
25    PEEKR    read destination value
26    X<> Z    
27    PEEKR    read source value
28    R^    lift for compare
29    X=Y?    are they equal?
30    SF 00    flag it so
31    RDN    undo lift
32    FS?C 00    equal values?
33    GTO 02    yes, skip writing
34    X<> T    position them crossed
35    POKER    write them back
36    RDN    
37    X<> Z    position them crossed
38    POKER    write them back
39    LBL 02    merge code
40    RDN    locate addreses in X, Y
41    E    
42    ST+ Z    increase adr-1
43    +    increase adr-2
44    DSE M    decrease counter
45    GTO 01    loop back if not done
46    "DONE"    block is done
47    AVIEW    
48    END

Cheers,
ÁM

PS. Slightly improved code updated