AltairZ80 / Altair8800 SIMH - Is it possible to set memory range as Read Only?

[Joseph Corda]

After I have loaded a "bin/rom" file into memory location (say to addr. F800) is it possible to have that memory F800 to FDFF set as as read only ?

Using SIMH I'm trying to emulate a system in which I'm not able to write to a "ROM" memory range just like a "real" system. 

Any suggestions would be appreciated 
Joe

[Patrick Linstruth]

Using AltairZ80, you can add the “ROM” option to the end of your LOAD command. This will tell the simulator to treat what you just loaded as ROM. This will also work when loading HEX files with “-h”.

sim> load dbl.bin 0 rom

256 bytes [1 page] loaded at 0 [ROM].

sim> set cpu verbose

sim> show cpu

CPU     

        64KB, NOITRAP, 8080, AZ80, STOPONHALT

        NONBANKED, ALTAIRROM, VERBOSE,

       0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF [16k]

00000: RWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW

04000: WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW

08000: WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW

0C000: WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWR,

0x[02 03 04 05 08 09 0A 10 11 12 13 14 15 16 17 18 19 28 29 2A 2B 32 33 A0 A1 A2 A3 A4 A5 A6 A7 FD FE FF]

        MMU, NOSWITCHER, NOPO

sim> 

Altair8800 simulator does not have this option yet. I am planning on implementing it with PROTECT and UNPROTECT options, example:

sim>set ram prot=f8-fd

Would protect pages F8-FD and ignore memory writes.

sim>set ram unprot=f8-fd

Would unprotect pages F8-FD and allow memory writes.

I think this is more in line with how the original 8800 worked, although not many RAM boards supported PROTECT.

I will see about getting that added to Altair8800 simulator in the next week.

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[Joseph Corda]

Thanks Patrick that would be great,, as I mainly use Altair8800 SIMH these days.

It was funny I discovered this issue working on JoeMon and I used STA/SHLD in a few spots in my code .. but due to my ignorance the variables were defined within the F800-FCFF range.. So JoeMon

worked in the simulator, but once burned to real EPROM adn run on real hardware I started to get errors and odd behavior. 
After a quick review of my source It was obvious what was wrong and has been fixed !
Going forward with a protect./unprotect option will be a big help so please let me know when I can pull a Altair8800 update from your repo 

Joe

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[Patrick Linstruth]

I added the ROM option to LOAD when messing around with AMON. The first thing AMON does is figure out how much RAM is in the system by writing/reading at page boundaries. It assumes AMON is in ROM and doesn’t check if it loops past FF00 back to 0000. If the entire system is RAM, it will loop forever.

[Joseph Corda]

Interesting ..
Currently JoeMon is not so smart.. and as such It does not check RAM boundaries.  
Honestly at this point I expect a user will adjust the source (ORG, STACK,  SIO parms ..etc) as needed for their system and re-assemble. 

Note even my MemTest is currently destructive and in SIMH would clobber itself if you tell it to. 
In my next version I will consider adding better smarts into MemTest..and possibly a non-destructive MemTest (if ROM space allows) etc but that will be in version 1.1 ;-)

Joe

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