NextZXOS and NextBASIC memory allocation

Before we begin to elaborate on NextZXOS' memory usage, it should be mentioned that Standard memory management and MMU management are internally synchronised for most cases. Every time a 16K bank is being paged in, the equivalent MMU unit gets the 8K bank component of the larger 16K bank NextZXOS uses. As mentioned previously NextZXOS also supports allRAM mode where the ROM is paged out; this is mainly used by CP/M. With this information out of the way, let's see how NextZXOS uses the memory.

By default the first 9 RAM banks are used as follows:

Generally speaking, banks 9+ are always available to the programmer, and can be accessed using the BANK command, while banks 0 – 8 can be used with the following exceptions:

• Bank 0 can be used, only if CLEAR has set the RAMTOP to below 49152.
• Bank 2 can be used, only if CLEAR has set the RAMTOP to below 32768.
• Banks 1,3,4,6 can be used if the BANK 1346 USR command has been used.
• Banks 7 and 8 can never be used.
• Bank 5 can be used with caution.
• Banks 9, 10 and 11 can be used for other purposes if you aren't using Layer 2 or you have changed their assignments with the LAYER BANK command.

From the above, it is easy to surmise what the initial bank assignments are after boot:

In case you were thinking that this looks easy enough – I could page in any bank I want, don't! In actuality, NextZXOS and NextBASIC expect certain things to be in certain places at all times within the memory map which is organised in the following manner:

 DISP_FILE1                    COLOUR_FILE1           SYSVARS            CHANS
+------------------------------+----------------------+------------------+---------------------~
| Display File                 | Attributes           | System Variables | Channel Information ~
+------------------------------+----------------------+------------------+---------------------~
  16384/4000h                    22528/5800h            23296/5B00h        23734/5CB6h

    PROG                                  VARS       E_LINE                       WORKSP
+-~~+----+------------------------+---------+---+-----------------------+--+---+~~-+
| ~ |80h | NextBASIC Program      |Variables|80h| Command or Program    |NL|80h| ~ |
| ~ |    |                        |         |   | Line being Edited     |  |   | ~ |
+-~~+----+------------------------+---------+---+-----------------------+--+---+~~-+

 WORKSP                    STKBOT           STKEND  STACK                RAMTOP   UDG    P_RAMT
                                                     POINTER
+-------+--+---------------+---------------+-------+--------+-------------+--+----+--------------~
|INPUT  |NL| Temporary     | Calculator    | Spare | Machine| NextBASIC   |? |3Eh | User Defined ~
|Data   |  | Workspace     | Stack         |       | Stack  | Return Stack|  |    | Graphics     ~
+-------+--+---------------+---------------+-------+--------+-------------+--+----+--------------~

Fig. 48 – Memory map usage by NextBASIC