© 2000 Scenix Semiconductor, Inc. All rights reserved. 123 SX User’s Manual Rev. 3.1 www.scenix.com Chapter 3 Instruction Set This  example  performs  16-bit  subtraction  of  file  registers  0Ah-0Bh  from  file
registers  0Ch-0Dh.  For  this  example,  the  CF  bit  in  the  FUSEX  configuration
register is programmed to 0 in order to implement subtraction with borrow. The first “sub” instruction subtracts the contents of 0Ah from 0Ch and clears the C
bit if a borrow occurs out of bit 7, or sets the C bit otherwise. The second “sub”
instruction subtracts the contents of 0Bh from 0Dh with borrow-in using the C bit. This algorithm can also be implemented with the device in the default configuration
(with the CF bit set to 1 in the FUSEX register), although not as efficiently. For
example, you can do the low-order subtraction, test the carry bit, decrement file
register 0Dh if the carry bit is 0, and then do the high-order subtraction.