From BeebWiki
Jump to: navigation, search

ANDY is an area of extended RAM, present in the Model B+ and Master series. It is made from the top of shadow RAM (the bottom 20 KB holding the shadow screen), and paged into paged ROM space from location &8000, when the top bit of ROMSEL is set. The area is 12 KB in the B+, and 4 KB in the Master. Even so, the MOS will not search for languages or service ROM images in this space.

When the B+ was released this area did not have an official name, and was left free for the user with a warning that it may not be available in "other Acorn products". Code running in addresses &A000 to &AFFF of this area can 'see' and manipulate the display memory in shadow RAM. It is not clear whether such code is rediverted to main RAM when the shadow display is turned off.

In the Master it was reduced to 4 KB, entitled ANDY and allocated to the MOS for system variables. This freed pages of main memory for other purposes. The remaining 8 KB of shadow RAM appeared as HAZEL, overlaying the beginning of the MOS at &C000.

ANDY can be accessed through the paged ROM space as above, or by calling OSWORD &05 or OSWORD &06 and giving an address of the form &FFFExxxx.

-- beardo 22:49, 14 September 2007 (BST)