Intel 80C196NU, User Manual

Page: 94 / 471

5-21
MEMORY PARTITIONS
This listing shows the disassembled code:
2080H ;C814 | PUSH WSR
2082H ;B14814 | LDB WSR,#48H
2085H ;44E4E2E0 | ADD E0H,E2H,E4H
2089H ;B21814 | LDB WSR,[SP]
208CH ;65020018 | ADD SP,#02H
2090H ;F0 | RET
2091H ;C814 | PUSH WSR
2093H ;B14814 | LDB WSR,#48H
2096H ;44EAE8E6 | ADD E6H,E8H,EAH
209AH ;B21814 | LDB WSR,[SP]
209DH ;65020018 | ADD SP,#02H
20A1H ;F0 | RET
The C compiler can also take advantage of this feature if the “windows” switch is enabled. For
details, see the MCS 96 microcontroller architecture software products in the Development Tools
Handbook.
5.3.3 Windowing and Addressing Modes
Once windowing is enabled, the windowed locations can be accessed both through the window
using direct addressing and through its actual address using indirect or indexed addressing. The
lower register file locations that are covered by the window are always accessible by indirect or
indexed operations. To re-enable direct access to the entire lower register file, clear bits 6:0 of the
window selection register. To enable direct access to a particular location in the lower register file,
you may select a smaller window that does not cover that location.
When windowing is enabled:
•
a direct instruction that uses an address within the lower register file actually accesses the
window in the upper register file;
•
an indirect or indexed instruction that uses an address within either the lower register file or
the upper register file accesses the actual location in memory.
The following sample code illustrates the difference between direct and indexed addressing when
using windowing.
PUSHA ; Pushes the contents of WSR onto the stack
LDB WSR, #17H ; Selects window 17H, a 128-byte block
; (windows 0380-03FFH into 0080-00FFH)
; The next instruction uses direct addr
ADD 40H, 80H ; mem_word(40H)
←
mem_word(40H) + mem_word(380H)
; The next two instructions use indirect addr
ADD 40H, 80H[0] ; mem_word(40H)
←
mem_word(40H) + mem_word(80H +0)
ADD 40H, 380H[0] ; mem_word(40H)
←
mem_word(40H) + mem_word(380H +0)
POPA ; reloads the previous contents into WSR