ST92196A datasheet(38/268 Pages) STMICROELECTRONICS

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DEVICE ARCHITECTURE

ADDRESS SPACE EXTENSION (Cont’d)

2.6.2 Addressing 64-Kbyte Segments

This extension mode is used to address Data

memory space during a DMA and Program mem-

ory space during any code execution (normal code

and interrupt routines).

Three registers are used: CSR, ISR, and DMASR.

The 6-bit contents of one of the registers CSR,

ISR, or DMASR define one out of 64 Memory seg-

ments of 64 Kbytes within the 4 Mbytes address

space. The register contents represent the 6

MSBs of the memory address, whereas the 16

LSBs of the address (intra-segment address) are

given by the virtual 16-bit address (see Figure 15).

2.7 MMU REGISTERS

The MMU uses 7 registers mapped into Group F,

Page 21 of the Register File and 2 bits of the

EMR2 register.

Most of these registers do not have a default value

after reset.

2.7.1 DPR[3:0]: Data Page Registers

The DPR[3:0] registers allow access to the entire 4

Mbyte memory space composed of 256 pages of

16 Kbytes.

2.7.1.1 Data Page Register Relocation

If these registers are to be used frequently, they

may be relocated in register group E, by program-

ming bit 5 of the EMR2-R246 register in page 21. If

this bit is set, the DPR[3:0] registers are located at

R224-227 in place of the Port 0-3 Data Registers,

which are re-mapped to the default DPR's loca-

tions: R240-243 page 21.

Data Page Register relocation is illustrated in Fig-

ure 13.

Figure 15. Addressing via CSR, ISR, and DMASR

Fetching program

Data Memory

Fetching interrupt

instruction

accessed in DMA

instruction or DMA

access to Program

Memory

16-bit virtual address

22-bit physical address

6 bits

MMU registers

CSR

ISR

DMASR

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