EXOS 2.1 Constants

---

1. System Segment Addresses

The top the system segment (segment 0FFh) contains certain defined addresses which may be accessed by the user of EXOS. These are:

 0BFFFh   - USR_P3       | These are the four segments which
 0BFFEh   - USR_P2       | were paged in when EXOS was last
 0BFFDh   - USR_P1       | called.
 0BFFCh   - USR_P0       | Undefined when not in EXOS.

 0BFFA/Bh - STACK_LIMIT    Lower limit of the system stack
                           which devices may ues. Allows a
                           safety margin for interrupts.

 0BFF8/9h - RST_ADDR       Warm reset address, must be in 
                           page zero.

 0BFF6/7h - ST_POINTER     Z80 address of the status­line RAM.

 0BFF4/5h - LP_POINTER     Z80 address of the start of the
                           video line parameter table.

 0BFF3h   - PORTB5         Current contents of Z80 port 0B5h,
                           used if any bits are modified.
 
 0BFF2h   - FLAG_SOFT_IRQ  Flag set to non­zero value to
                           cause a software interrupt. The
                           value will become the software 
                           interrupt code.

 0BFF0/1h - SECOND_COUNTER 16­bit seconds counter, never read
                           by EXOS or built in devices.

 0BFEFh   - CRDISP_FLAG    Flag can be set non­zero by an
                           extension ROM when it is called to
                           be initialised (action code 8) to
                           suppress the initial flashing
                            display.

 0BFED/Eh - USER_ISR       Address of user interrupt routine,
                           must be in page 0. 0000h for no
                           user interrupt routine.  

Next Chapter: EXOS Function Codes.