Fig. 5 Stack Registers (Sks) Structure; Fig. 6 Example Of Operation At Subroutine Call - Renesas 4513 User Manual

4500 series 4-bit single-chip microcomputer

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HARDWARE

FUNCTION BLOCK OPERATIONS

(5) Stack registers (SK

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.

1-18

) and stack pointer (SP)

4513/4514 Group User's Manual

Program counter (PC)

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 SK

When the BM instruction is executed after eight

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

and the contents of SK