ST9 datasheet(23/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

To address the 4 Mbytes of memory, the address bus is 22 bits wide. To manage the 22-bit

address with 16-bit direct or indirect addressing, the memory mechanism adds extra bits to the

16-bit address and then works with segments (see Figure 10) or pages (see Figure 13). The

memories are arranged in 64 segments of 64 Kbytes for the program and in 256 segments of

16 Kbytes for data.

A set of special registers is used to extend the 16-bit address. Programs use the CSR, DMA

uses the DMASR or ISR and interrupts use the ISR or CSR register to provide the 6 Most Sig-

nificant Bits to make a 22-bit address. Data uses a set of four registers (DPR0-3) to provide

the 8 Most Significant Bits to make a 14-bit address.

Data can be addressed in the Program segment by using special move instructions: lddp,

ldpp

, ldpd, lddd.

It is easier from a hardware point of view to use only one address space for Program and

Data.

Figure 10

. Addressing via CSR, ISR and DMASR

CSR

DMASR

ISR

16-bit Virtual Address

22-bit Physical Address

6 bits

Fetching program

instruction

Data Memory

accessed in DMA

Fetching interrupt

instruction or DMA

access to Program

Memory

MMU Registers

ST9 Datasheet(PDF) 23 Page - STMicroelectronics