7906 datasheet(25/531 Pages) RENESAS

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7906 Group User’s Manual Rev.2.0

CENTRAL PROCESSING UNIT (CPU)

2.1 Central processing unit (CPU)

2-4

2.1.4 Stack pointer (S)

The stack pointer (S) is a 16-bit register. It is used for a subroutine call or an interrupt. It is also used when

addressing modes using the stack are executed. The contents of S indicate an address (stack area) for

storing registers during subroutine calls and interrupts. Bank 016 is specified for the stack area. (Refer to

section “2.3 Access space.”)

When an interrupt request is accepted, the microcomputer stores the contents of the program bank register

(PG) at the address indicated by the contents of S and decrements the contents of S by 1. Then the

contents of the program counter (PC) and the processor status register (PS) are stored. The contents of

S after accepting an interrupt request is equal to the contents of S decremented by 5 before accepting of

Fig. 2.1.2 Contents of stack area after accepting

interrupt request

q “S” is the initial address that the stack pointer (S)

indicates at accepting an interrupt request.

The S’s contents become “S – 5” after storing the

above registers.

Address

S–4

S–3

S–2

S–1

Stack area

S–5

Processor status register’s low-order byte (PSL)

Processor status register’s high-order byte (PSH)

Program counter’s low-order byte (PCL)

Program counter’s high-order byte (PCH)

Program bank register (PG)

the interrupt request. (See Figure 2.1.2.)

When completing the process in the interrupt routine

and returning to the original routine, the contents of

registers stored in the stack area are restored into

the original registers in the reverse sequence

(PS

→PC→PG) by executing the RTI instruction. The

contents of S is returned to the state before accepting

an interrupt request.

The same operation is performed during a subroutine

call, however, the contents of PS is not automatically

stored. (The contents of PG may not be stored.

This depends on the addressing mode.)

During interrupts or subroutine calls, the other

registers are not automatically stored. Therefore, if

the contents of these registers need to be held on,

be sure to store them by software.

Additionally, the S’s contents become “0FFF16” at

reset. The stack area changes when subroutines

are nested or when multiple interrupt requests are

accepted. Therefore, make sure of the subroutine’s

nesting depth not to destroy the necessary data.

❈ Refer to “7900 Series Software Manual” for addressing modes and instructions.

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