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ST9 USER GUIDE

These operators are especially useful when applied to a function or instruction label, although

the macro-assembler and assembler do not verify the type of the label.

Example:

r0,#SEG Function

; extract the 6-bit segment

ldw

rr2,#SOF Function

; extract the 16-bit offset

calls

(r0),(rr2)

The same functions exist in C language: SEG(Function); SOF(Function).

3.1.7.2 Interrupt Service Routine Segment

One program segment is reserved for storing the Interrupt Service Routines. All the interrupt

routines start in this segment.

To obtain a 22-bit address, the 16-bit address is concatenated with 6 bits from the ISR register

(the 6 bits from the ISR register are the high-order bits of the address).

To offer compatibility with the previous ST9 versions and to have a new powerful address

mechanism, you can select “ST9” mode using the EMR2 bit in the ENCSR register.

Both modes use the concatenation of the ISR register value and the 16-bit address as shown

in Figure 10 to address the interrupt vector.

Then, in “ST9” mode, only the ISR register value is used during the interrupt routine. So it’s not

possible to jump to another segment from the Interrupt Service Routine because the CSR reg-

ister value is not saved with the FLAGR value and the current PC value when the interrupt oc-

curs. The advantage of “ST9” mode is to reduce stack memory usage and CPU cycles by not

saving the CSR value (see Figure 11). This figure shows you the different mechanisms that

are used when an interrupt occurs, when a branch or a call is executed during the interrupt

routine and when the return from interrupt instruction (RETI) is executed.

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