8XC196MC, MD, MH USER’S MANUAL

4.2WINDOWING

Windowing expands the amount of memory that is accessible with register-direct addressing. Register-direct addressing can access the lower register file with short, fast-executing instruc- tions. With windowing, register-direct addressing can also access the upper register file and pe- ripheral SFRs.

Windowing maps a segment of higher memory (the upper register file or peripheral SFRs) into the lower register file. The window selection register (WSR) selects a 32-, 64-, or 128-byte seg- ment of higher memory to be windowed into the top of the lower register file space. Figure 4-2 illustrates a 128-byte window.

128-byte Window

(WSR = 13H)

WSR Window in

Lower Register File

8XC196MC,MD

02FFH

01FFH

0180H

00FFH

0080H

0000H

128-byte Window

(WSR = 13H)

WSR Window in

Lower Register File

8XC196MH

A3062-01

Figure 4-2. Windowing

NOTE

Memory-mapped SFRs must be accessed using indirect or indexed addressing modes; they cannot be accessed through a window. Reading a memory- mapped SFR through a window returns FFH (all ones), and writing to a memory-mapped SFR through a window has no effect.

4-12

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Intel 8XC196MD manual Windowing, 8XC196MC,MD, 02FFH 01FFH, 00FFH, 8XC196MH