### 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: | Bank | Description | Address Range | |---|---|---| | 0 | Standard 48K Spectrum memory | 49152 – 65535 | | 1 | RAMdisk | | | 2 | Standard 48K Spectrum memory | 32768 – 49151 | | 3 | RAMdisk | | | 4 | RAMdisk | | | 5 | Standard 48K Spectrum memory | 16384 – 32767 | | 6 | RAMdisk | | | 7 | Used for workspace and data structures by *NextZXOS* | | | 8 | Used for additional screen data (for *LoRes*, *HiRes* and *HiColour*) and other data by *NextZXOS* | | | 9 – 111 | Available for user programs (By default banks **9**,**10** and **11** are used by *Layer 2*) | | 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: | Slot 1 | Slot 2 | Slot 3 | Slot 4 | |---|---|---|---| | **ROM** | **Bank 5** | **Bank 2** | **Bank 0** | 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*