Macintosh 128K/512K Technical Details - Memory Map

Memory Map

• RAM ($000000 - $07FFFF) is divided up into a series of different functional areas:
  • System globals ($000000 - $000AFF)
  • System heap ($000B00). SysZone points to start, ApplZone points to end + 1
  • Application heap (ApplZone; grows upwards. HeapEnd points to its end; ApplLimit sets maximum)
  • Stack. Grows downwards from CurStackBase; SP = A7 points to top of stack.
  • QuickDraw globals. (206 bytes) A5 points to boundary between QD globals and App globals (the "A5 world").
  • Application globals
  • Application parameters (32 bytes)
  • Jump table
  • Alternate screen buffer, 21,888 bytes (BufPtr)
  • 9344 bytes of undocumented space
  • 740 bytes alternate sound buffer
  • 796 bytes undocumented
  • Screen buffer, 21,888 bytes (ScrnBase = $01A700)
  • System Error handler, 128 bytes
  • Main sound buffer, 740 bytes
  • 28 bytes undocumented, MemTop points to the end of RAM, +1
• ROM ($400000 - $41FFFF)
• sccRBase - SCC read operations - $9FFFF8
• sccWBase - SCC write operations - $BFFFF9
• IWM (dBase) $DFE1FF
• VIA (vBase) $EFE1FE
  • aVBufB - register B base - $EFE1FE
  • aVBufA - register A base - $EFFFFE
  • aVIFR - interrupt flag register - $EFFBFE
  • aVIER - interrupt enable register - $EFFDFE

Note that the RAM map is organised so that the system globals, system and application heaps grow upwards from low memory, everything else grows downwards from MemTop, from high memory towards low memory. On the 512K Macintosh, the "extra" RAM thus appears as a wider gap between the application heap and the stack, where it is available for application use.