TMP88CS38 datasheet(28/226 Pages) TOSHIBA

28 / 226 page

TMP88CS38/CM38A/CP38A

2007-09-12

88CS38-28

(2) Saving/restoring general-purpose registers

During interrupt acceptance processing, the program counter (PC) and the program

status word (PSW) are automatically saved on the stack, but not the accumulator and

other registers. These registers are saved by the program if necessary. Also, when

nesting multiple interrupt services, it is necessary to avoid using the same data

memory area for saving registers.

The following method is used to save/restore the general-purpose registers.

1. General-purpose register save/restore by automatic register bank changeover

The general-purpose registers can be saved at high speed by switching to a

the banks 1 to 15 are assigned to interrupt service tasks. To increase the efficiency

of data memory utilization, the same bank is assigned for interrupt sources which

are not nested.

The switched bank is automatically restored by executing an interrupt return

instruction [RETI] or [RETN]. Therefore, it is not necessary for a program to save

the RBS.

Example: Register bank changeover

PINTxx:

Interrupt processing

RETI

VINTxx:

PINTxx

;

RBS

← RBS + 1

2. General-purpose register save/restore by register bank changeover

The general-purpose registers can be saved at high speed by switching to a

the banks 1 to 15 are assigned to interrupt service tasks.

Example: Register bank changeover

PINTxx:

LD RBS, n

Interrupt processing

RETI

;

Restores bank and returns

VINTxx:

PINTxx

;

Interrupt service routine entry address

Figure 1.5.4 Saving/Restoring General-purpose Registers

Acceptance

of interrupt

Saving

registers

Restoring

registers

Interrupt

service task

Interrupt return

Main task

(b) Saving/restoring using push/pop or data transfer instructions

Acceptance

of interrupt

Bank m

Interrupt

service task

Restore to bank m

automatically by

[RETI]/[RETN]

Interrupt return

Switch to bank n by

LD, RBS and n

instruction

Switch to bank n

automatically

Main task

(a) Saving/restoring by register bank changeover

Time

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