Hi

Some rather basic questions:

How much RAM extra do you have on a Master? Is it 128K on all models? You can do double buffering too, right?

Does the CPU run any faster?

Are there any other major differences?

Yep - 32k main RAM, 20k shadow RAM (3000-7FFF), 8k FS RAM (C000-DFFF), 4k VDU RAM (8000-8FFF) and 4 banks of sideways RAM (banks 4-7). It is possible to disable the sideways RAM (needed to use the internal ROM sockets) but it's very rare to see this done.

The screen can read from main memory or shadow memory, and either can be mapped to 3000-7FFF.



No, but it does have a few new instructions (it's a 65C12 if you want to look it up). STZ, PHX/PHY/PLX/PLY, INC A and DEC A, BIT immediate, and bit test and set/reset TRB and TSB.



Lots of differences in the OS (eg don't rely on any tables in the OS ROM). Also PAGE is always at &E00.

Regarding the OS, the main differences (which can cause OS 1.20 games to fail) are:

* The character set isn't at &C000...&C2FF in the OS. Instead, a copy exists in RAM at (I think) &8900 in the VDU RAM bank (which has to be paged in by setting bit 7 of the paged ROM register at &FE30). This means that some games, e.g. Knight Lore, which try to plot characters directly from these definitions, get it all wrong. Luckily, &C000...&C2FF is normally paged as RAM on the Master, so you can take a copy of the character set and put it there. Blurp does this actually

* As Tom said, there are no lookup tables for multiplying by 640 (to get screen row addresses). This means you can't install your own lookup table for custom screen widths and use the regular VDU drivers to write text. This shouldn't really be a problem, but some games rely on it (e.g. Contraption).

* The VDU variables in &300...&380 (and also the zero page ones) are used slightly differently (to accommodate extra graphics features in the Master OS). This causes the daggers in Frak to appear and disappear apparently randomly for some reason.

* The disc controller hardware is mapped to a different location to that of the Beeb (and of couse is a 1770 controller rather than 8271), but this shouldn't really matter.

* All of the graphics plotting routines live in the OS ROM between &C000...&DFFF. There is a curious memory access mode the Master can be put into which basically says: "If memory in the range &3000...&7FFF (screen memory) is accessed by an instruction executed from an address in the range &C000...&DFFF, force it to access from main RAM (or shadow RAM), regardless of whichever is physically mapped to that address range". This is how shadow RAM can be written to by the VDU drivers without needing to actually page it in.

I can't think of any others for the moment, apart from other hardcoded addresses from OS 1.20 which changed on the Master (and which shouldn't be relied on anyway). None of these things should particularly pose a problem if you're doing things 'properly' (or doing everything yourself through the hardware).

Excellent.. very useful cheers!

I'm planning on doing a "deluxe" version of Mountain Panic on the Master, with extra graphics and levels. Possibly with music if I can convince Tom to help me

Great! I expect this'll be very well received.

Cheers Trev I think the Master will allow me to do the version I want to, so with a bit of luck should turn into something quite special!

There seems to be some difference in the ADC on the Master, too. I've put in support for joysticks which works fine and dandy on the humble BBC, but on the Master it doesn't work at all.

Edit: I've just by luck found out why - the ADC on the Master is at a different SHEILA address (Page 154 of the New Advanced User Guide).

Just a suggestion, but would it perhaps be possible to break the game up Imogen-style on a standard Beeb? Maybe that way the Beeb version would still get all the gameplay, music, etc. of the Master version.

Hmm not really no... it's not broken up into distinct puzzles like Imogen.

I want to make a game similar in style to Citadel and The Two Towers (which was what sparked my interest).

Still amazed at how he crammed it all into Citadel. Top job.

Yes, I agree Citadel is very, very clever. I spent all night once figuring out how Michael Jakobsen got the main character in Citadel climbing up gentle slopes...

Citadel and Repton 2 are probably my favourite BBC Micro games.

This also applies to addresses &A000..&AFFF in ANDY on the B+. (B+ User Guide, p.452.) What actually appears between the top of ANDY in the Master (&9000) and HAZEL?

--Greg

Whatever ROM is mapped in. ROMSEL is the low 4 bits of &FE30, and bit 7 of that address maps in ANDY at &8000-&8FFF over the ROM.

This memory mode is more important on the B+ than the Master as it's the _only_ way the shadow memory can be accessed. Another fun problem is that it's impossible to access shadow memory without displaying it, as I recall.

Thanks Tom. I was hoping it would be the bottom of the shadow screen.