ST9 datasheet(27/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

Figure 12

. Interrupt Processing in “ST9” Mode

3.1.7.3 DMA Segment

To address the total 4 Mbytes of memory in a DMA transaction, the DMASR points to a

64-Kbyte segment. Since there is no need to have more than one segment at the same time

for the transaction, the DMA uses a single 64-Kbyte segment instead of 4 pages like a Data

Segment (see below). To use the DMASR register, the DP bit in the DMA Address Register

(DAPR) must be set. If DP is reset, the DMA uses the ISR register instead of the DMASR reg-

ister.

CSR

PC MSB

PC LSB

STACK

16-bit

22-bit

6-bit

16-bit

16-bit Address

of any branch

Addresses of local and far

branches (to any segment) are

extended to 22-bits as shown

CSR

PC MSB

PC LSB

STACK

22-bit

16-bit

FLAGR register

Return from interrupt.

The current PC value and the

CSR value saved in the stack are

concatenated to make the 22-bit

FLAG

CSR

ISR

Before loading the CSR register with

the ISR value, the CSR value is saved

in the stack.

CSR

is saved with the current PC value

in the stack and the CSR

6-bit

CSR

FLAG

return address.

CSR+PC

represent

the 22-bit

current address

saved.

CSR

PC MSB

PC LSB

STACK

FLAG

segment to jump to.

CSR

PC MSB

PC LSB

Next segment for

a far branch

Interrupt

When an interrupt occurs, the 16-bit

Interrupt Vector is extended to 22-bits

as shown above.

22-bit

6-bit

8 bits

Interrupt

Service

Routine

Address

For a far CALL, the CSR value

ISR

IVR

at ’0’

6-bit

22-bit

Vector

above.

8-bit

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