4508 datasheet(15/142 Pages) RENESAS

15 / 142 page

4508 Group

Rev.1.02

2006.12.22

page 15 of 140

REJ03B0148-0102

(5) Stack registers (SKS) and stack pointer (SP)

Stack registers (SKs) are used to temporarily store the contents of

program counter (PC) just before branching until returning to the

original routine when;

• branching to an interrupt service routine (referred to as an interrupt

service routine),

• performing a subroutine call, or

• executing the table reference instruction (TABP p).

Stack registers (SKs) are eight identical registers, so that subrou-

tines can be nested up to 8 levels. However, one of stack registers is

used respectively when using an interrupt service routine and when

executing a table reference instruction. Accordingly, be careful not to

over the stack when performing these operations together. The con-

tents of registers SKs are destroyed when 8 levels are exceeded.

The register SK nesting level is pointed automatically by 3-bit stack

pointer (SP). The contents of the stack pointer (SP) can be trans-

ferred to register A with the TASP instruction.

Figure 5 shows the stack registers (SKs) structure.

Figure 6 shows the example of operation at subroutine call.

(6) Interrupt stack register (SDP)

Interrupt stack register (SDP) is a 1-stage register. When an interrupt

occurs, this register (SDP) is used to temporarily store the contents

of data pointer, carry flag, skip flag, register A, and register B just be-

fore an interrupt until returning to the original routine.

Unlike the stack registers (SKs), this register (SDP) is not used when

executing the subroutine call instruction and the table reference in-

struction.

(7) Skip flag

Skip flag controls skip decision for the conditional skip instructions

and continuous described skip instructions. When an interrupt oc-

curs, the contents of skip flag is stored automatically in the interrupt

stack register (SDP) and the skip condition is retained.

Fig. 5 Stack registers (SKs) structure

Fig. 6 Example of operation at subroutine call

SK0

SK1

SK2

SK3

SK4

SK5

SK6

SK7

(SP) = 0

(SP) = 1

(SP) = 2

(SP) = 3

(SP) = 4

(SP) = 5

(SP) = 6

(SP) = 7

Program counter (PC)

Executing RT

instruction

Executing BM

instruction

Stack pointer (SP) points “7” at reset or

returning from RAM back-up mode. It points “0”

by executing the first BM instruction, and the

contents of program counter is stored in SK0.

When the BM instruction is executed after eight

stack registers are used ((SP) = 7), (SP) = 0

and the contents of SK0 is destroyed.

Returning to the BM instruction execution

address with the RT instruction, and the BM

instruction becomes the NOP instruction.

(SP)

← 0

(SK0)

← 000116

(PC)

← SUB1

Main program

000216 NOP

Address

000016 NOP

000116 BM SUB1

Subroutine

SUB1 :

NOP

(PC)

← (SK0)

(SP)

← 7

Note :

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