Spectrum memory map when running BBCBasic or BBC MOS
         ====================================================

Memory Layout Overview
----------------------
&0000   ROM
&4000   Screen
&4000   Lowest possible Low Water Mark with Shadow Screen
&5B00   Spectrum printer buffer - may be usable as workspace
&5C00   Spectrum workspace
&6000   Lowest possible Low Water Mark without Shadow Screen
&FF00   Highest possible High Water Mark for BBC environment
&FF80   BBC MOS Interface addresses


Spectrum Memory Map
-------------------
&0000   Onboard Rom
&4000   Screen bitmap
&5800   Screen colourmap
&5B00   Previously printer buffer
&5C00   Spectrum system variables
&5CB6   Interface 1 System Variables
&5CF0   A single microdrive map
          NB: Microdrive LOAD pushes 32 byte map onto stack
&5D10   Start of CHANS
          standard channels extend to:
&5D24
          a single microdrive channel would extend to:
&5F77   &80 byte, terminating CHANS area
&5F78   &80 PROG, VARS
&5F79   &0D E_LINE
&5F7A   &80
&5F7B   WORKSP, STKBOT, STKEND

 129 bytes of workspace for Spectrum routines
 Spectrum will never give 'Out of memory' error, as
 SP is always above &6000, at or below HIMEM
 Eg, Microdrive LOAD pushes 32 byte map onto stack,
 this will go onto stack at HIMEM, around &F300

&5FFD   End of spare space
&5FFC       ERR_SP
&5FFE   &00
&5FFF   &3E RAMTOP - End of Spectrum memory area


BBC BASIC Memory use
====================
&6000   Default PAGE
&F000   Default HIMEM

&F000   BBC BASIC String buffer
&F100   BBC BASIC Comand buffer
&F200   System variables
&F4CE   Control block used by Basic
 &F4CE-F  => String buffer
 &F4D0    Max char value
 &F4D1    Min char value
 &F4D2    Max input string length
 &F4D3    Last byte of longest control block used


BBC MOS Memory use
==================
&F200  CLICOM input buffer (255 bytes max - up to &FF)

&F300  NMI Code Space
 &F300-1 Initialised to RETN
 &F37F   End of NMI code space

&F380  Reserved for sideways ROM access
       Could be small transient command buffer up to &F3FF

&F3E0  Error buffer - Spectrum errors translated into strings (up to 30 bytes)
       Temporary stack from &F3FF downwards

&F400  VDU workspace
 &F400  UDCs
 &F480  VDU queue and workspace

&F490  ROM table

&F4A0  MOS workspace
 &F4A0-1  Ticker
 &F4A2-3  CopyAddr
 &F4A4-5
 &F4A6-7

&F4A8  Filing system workspace
 &F4A8    CURDRV - Current filing system
 &F4A9    CATFLG
 &F4AA    
 &F4AB    

&F4AC  Tape Load Header
 &F4AC    Filetype
 &F4AD-B5 Filename
 &F4B7-8  Length
 &F4B9-A  LOAD
 &F4BB-C  EXEC

&F4BD  Tape Save Header, Tape Search Header
 &F4BD    Filetype
 &F4BE-C7 Filename
 &F4C8-9  Length
 &F4CA-B  LOAD
 &F4CC-D  EXEC

&F4CE  Control block, eg for OsFile:
 &F4CE-F  => Filename
 &F4D0-3  LOAD
 &F4D4-7  EXEC
 &F4D8-B  Length/Start
 &F4DC-F  Attrs/End

&F4C6  Enough space in ROM for MOS code
&F4E0  MOS code


BBC MOS Interface Addresses
---------------------------
&FF80   Escape flag
&FF81   TEMPA
&FF82   FAULT
&FF84   DEFERR
&FF86   LPTR        Command line tail pointer
&FF88   MEMBOT      Bottom of available memory
&FF8A   MEMTOP      Top of available memory
&FF8C   ROMPORT     (ROM paging port - now unused)
&FF8E   SCREEN      Timex display setting, (port &FF value)*2, b7=1 - no Timex screen
&FF8F   ROMSEL      Currently selected ROM
&FF90   ZXROM       Value to send to ROMPAGE to select ZXROM
&FF91   BBCROM      Value to send to ROMPAGE to select BBCROM
&FF92   ROMPAGE     Page bottom 16K of memory
&FF95   SERVICE
&FF98   _BDOSRun    Cold start entry point
&FF9B   _BDOSWarm   QUIT

&FF9E  [PR_OUT]
&FFA1   RD_DEC
&FFA4   RD_HEX
&FFA7   OSQUIT      Exit current environment
&FFAA   PR_HEX      Print A in hex
&FFAD   PR2HEX      Print HL in hex
&FFB0   USERINT
&FFB3   PR_TEXT     Print inline text until NOP
&FFB6  [PRNT_C]
&FFB9   CLICOM
&FFBC   ERRJMP
&FFBF   INITERR
&FFC2  [SEEK_0]
&FFC5  [TSTKBD]
&FFC8  [TERM]
&FFCB   OSFSC
&FFCE   OSFIND
&FFD1   OSGBPB
&FFD4   OSBPUT
&FFD7   OSBGET
&FFDA   OSARGS
&FFDD   OSFILE
&FFE0   OSRDCH
&FFE3   OSASCI
&FFE7   OSNEWL
&FFEC   PRNTCR
&FFEE   OSWRCH
&FFF1   OSWORD
&FFF4   OSBYTE
&FFF7   OS_CLI
&FFFA   BRKV
&FFFC   EVENTV
&FFFE   IRQV

ROM Paging Values
-----------------
+---+---+---+---+---+---+---+---+
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---+---+---+---+---+---+---+---+
          |       +---+---+---+----- Timex MMU bits 2-5
          |
          +--- 0 : b7-b4 used to page ROMs with &7FFD
          +--- 1 : b4=0: DOCK paged in, b4=1: EX paged in

On the Spectrum SE with no ROM expansion, this gives the following
values:

  &00 - HOME ROM 0
  &10 - HOME ROM 1
  &20 - DOCK
  &30 - EX