Executing FOCAL programs from ROM vs. RAM

[Ángel Martin]

(04-11-2019 04:41 AM)hth Wrote:  Not all ROM addresses are possible for user code. The lower addresses are set aside for the internal ROMs, no user code exists in them. The 17 bit PC is compressed to 16 bits when stored on the stack, taking advantage of this. A compressed RAM address uses 12 bits in the return stack (plus a 0 upper nibble), a ROM address is 16 bits with a non-zero upper nibble.

Is that so? I may not fully understand your comment about the lower addresses, how low do you mean? FOCAL programs run happily on the printer ROM (page 6) and also in the Library#4 (page 4).

I need to ruminate your comments on the 17-bit compressed to 16; as far as I knew the RAM addresses are in the format "BRRR", i.e. always 16 bits. But you know this much better than me, your RAMED function is to this day the best implementation known to man on the subject!

ÁM

PS. I admit that the addresses game is one of my least well understood, but I seem to remember that when we were experimenting on an extension of the PC (using an multi-byte XEQ/RTN) we were close to coming up with a working model...