4513 datasheet(31/210 Pages) RENESAS

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1-18

HARDWARE

4513/4514 Group User’s Manual

FUNCTION BLOCK OPERATIONS

(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 inter-

rupt 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 care-

ful not to over the stack when performing these operations

together. The contents of registers SKs are destroyed when 8 lev-

els 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

transferred 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 inter-

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

contents of data pointer, carry flag, skip flag, register A, and regis-

ter B just before 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 refer-

ence instruction.

(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

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 :

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.

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