PDP11 BBC BASIC Memory overview
                       ===============================

        0000  +--------------------------------------------+
              | BASIC interpreter code                     |
              +============================================+  0000 -----------------+
              | TTY settings                      16 bytes |                        |
              +--------------------------------------------+  0010                  |
              | Keyboard workspace                12 bytes |                        |   Unix
              +--------------------------------------------+  001C                  | interface
              | TRAP buffer and CHAROUT buffer    10 bytes |                        |
              +--------------------------------------------+  0036                  |
              | MEMBOT, MEMTOP                     4 bytes |                        |
              +============================================+  003A = &3A bytes  ----+
              | Any future expansion will be here          |                        |
              +--------------------------------------------+ -0006                  |
              | RND workspace                      6 bytes |                        |
              +--------------------------------------------+  0000 <-- always ---+  |
              | String buffer                    256 bytes |         PAGE-&300   |  |
              +--------------------------------------------+  0100               |  |
              | Command buffer + MOS Buffer      256 bytes |                     |  |
              +--------------------------------------------+  0200                  |
              | Integer variables                108 bytes |                        |
              +--------------------------------------------+  026C                  |
              | Variable pointers                116 bytes |                        |
              +--------------------------------------------+  02E0                  |
              | Memory pointers                   12 bytes |                        |
              +--------------------------------------------+  02EC                  |
              | Program lines                      8 bytes |                        |
              +--------------------------------------------+  02F4                  |
              | Program pointers and flags        12 bytes |                        |
              +============================================+  0300 = &306 bytes  ---+
        PAGE  | Program text                               |  
              |                                            |
        TOP   +--------------------------------------------+
        LOMEM | Variables                                  |
              |                                            |
              +--------------------------------------------+
        END   | Unused memory                              |
              |                                            |
              +--------------------------------------------+
        r6    | Stack                                      |
              |                                            |
              +--------------------------------------------+
        HIMEM |                                            |
              | PDPTube                Unix                |
              +----------------------+---------------------+
              | FExx  command line   | xx00  end of memory |
              +----------------------+        eg E000      |
              | FF00  error buffer   | FC00                |
              | FFFF  default escflg | FFxx  command line  |
        FFFF  +----------------------+---------------------+

   Internal memory:
     ^@%-512 : String buffer, string can be fetched with $(^@%-512)
     ^@%-256 : Command buffer, string can be fetched with $(^@%-256)
     ^@%     : Integer variables
     ^@%+108 : Pointers to dynamic variables


                           PDP11 BBC BASIC Memory details
                           ------------------------------

                    +--------------------------------------------+
                    | Unix interface variables                   |
                    |                                            |
                    | additional entries may go here             |
                    |                                            |
                    | TTY settings                    - 16 bytes |  may or
                    | SV_TTY     - Current TTY settings          |   may 
                    | SV_OSTTY   - Host's tty flags              |  not be 
                    | SV_IOCTL   - Current tty characters        | present
                    | SV_OSIOCTL - Host's tty Intr+Quit chars    |
                    |                                            |
                    | Keyboard workspace              - 12 bytes |
                    |                                            |
                    | TRAP I/O buffers                - 10 bytes |  may or
                    | TRAP_BUF   - TRAP and four parameters      |   may 
                    | CHAR_BUF   - Character I/O buffers         |  not be 
                    |                                            | present
^@%-&20C            | MEMBOT     - absolute memory limits        |
^@%-&20A            | MEMTOP                          -  4 bytes |
                    +--------------------------------------------+
                    | BASIC workspace                            |
                    |                                            |
                    | additional entries may go here             |
                    |                                            |
^@%-&206  PAGE-&306 | SV_RAND    - RND seed                      |
^@%-&202  PAGE-&302 | SV_RAND4   - Fifth byte of RND seed        |
^@%-&201  PAGE-&301 | SV_STEP    - AUTO step                     |
                    |                                            |
^@%-&200  PAGE-&300 | String buffer                  - 256 bytes |
^@%-&100  PAGE-&200 | Command buffer                 - 256 bytes |
^@%-&012  PAGE-&112 | MOS_BUF                                    |
^@%       PAGE-&100 | Integer variables              - 108 bytes |<--+
^@%+&06C  PAGE-&094 | Pointers to dynamic variables  - 116 bytes |   |
                    |                                            |   |
                    | Memory pointers                 - 12 bytes |   |
^@%+&0E0  PAGE-32   | SV_PAGE    - PAGE  - start of program      | fixed
^@%+&0E2  PAGE-30   | SV_TOP     - TOP   - end of program        |  &100
^@%+&0E4  PAGE-28   | SV_LOMEM   - LOMEM - start of variables    |  bytes
^@%+&0E6  PAGE-26   | SV_VAREND  - END   - end of variables      |   |
^@%+&0E8  PAGE-24   | SV_STACK   - r6    - bottom of local stack |   |
^@%+&0EA  PAGE-22   | SV_HIMEM   - HIMEM - top of stack/memory   |   |
                    |                                            |   |
                    | Program lines                    - 8 bytes |
^@%+&0EC  PAGE-20   | SV_LINE    - Current line number           |
^@%+&0EE  PAGE-18   | SV_TRACE   - TRACE line number             |
^@%+&0F0  PAGE-16   | SV_AUTO    - AUTO line number              |
^@%+&0F2  PAGE-14   | SV_ERL     - Line number error occured in  |
                    |                                            |
                    | Program pointers                 - 6 bytes |
^@%+&0F4  PAGE-12   | SV_ONERR   - Points to after ON ERROR      |
^@%+&0F6  PAGE-10   | SV_FAULT   - Last error                    |
^@%+&0F8  PAGE-8    | SV_DATA    - DATA pointer                  |
                    |                                            |
                    | Program settings                -  6 bytes |
^@%+&0FA  PAGE-6    | SV_COUNT   - COUNT                         |
^@%+&0FB  PAGE-5    | SV_WIDTH   - WIDTH                         |
^@%+&0FC  PAGE-4    | SV_OPTIONS - LISTO and OPT options         |
^@%+&0FD  PAGE-3    | SV_ERR     - ERR                           |
^@%+&0FE  PAGE-2    | SV_SYS     - I/O flags                     |   |
^@%+&0FF  PAGE-1    | SV_ESCFLG  - Escape flag                   |   |
                    +--------------------------------------------+   |
^@%+&100  PAGE      |  Approx &3800 on PDPTube                   | --+
                    |         &4000 on Unix                      |
                    |                                            |
          TOP       +--------------------------------------------+
          LOMEM     | Variables                                  |
                    |                                            |
                    +--------------------------------------------+
          END       | Unused memory                              |
                    |                                            |
                    +--------------------------------------------+
          r6        | Stack                                      |
                    |                                            |
                    +--------------------------------------------+
          HIMEM     |                                            |
                    | PDPTube                Unix                |
                    +----------------------+---------------------+
                    | FExx  command line   | xx00  end of memory |
                    +----------------------+        eg E000      |
                    | FF00  error buffer   | FC00                |
                    | FFFF  default escflg | FFxx  command line  |
          FFFF      +----------------------+---------------------+