© 2000 Scenix Semiconductor, Inc. All rights reserved. 47 SX User’s Manual Rev. 3.1 www.scenix.com Chapter 2 Architecture If you  attempt to  nest  subroutines beyond eight levels,  or if you execute a return-from-subroutine
instruction without a prior corresponding “call” instruction, unpredictable results will occur because
an incorrect address will be copied to the program counter. The stack is not used for interrupt processing and is therefore not involved in the return-from-interrupt
instructions (RETI and RETIW). For information on interrupt processing, see Chapter 6. 2.8 Device Configuration Options The SX device has three 12-bit configuration registers that can be read or written at the same time that
the instruction memory is programmed: • FUSE word register, accessible by a device programming command • FUSEX word register, accessible by a device programming command • DEVICE word register, a read-only word accessible by a device programming command These registers are not accessible to the application program at run time. They can only be read or
written when the device is set up for programming the instruction memory. The  register  formats  are  shown  in  Figure 2-7  and  the  configuration  fields  within  the  registers  are
explained in Table 2-9, Table 2-10, Table 2-11 and Table 2-12. Note that the format of the FUSEX
register depends on the SX device type (SX18/20/28AC and SX18/20/28AC75 or SX48/52BD).