M34282E2GP datasheet(7/69 Pages) RENESAS

7 / 69 page

Rev.1.33

Mar 18, 2004

page 7 of 67

4282 Group

Fig. 5 Stack registers (SKs) structure

Fig. 6 Example of operation at subroutine call

(5) Most significant ROM code reference enable flag (URS)

URS flag controls whether to refer to the contents of the most

significant 1 bit (bit 8) of ROM code when executing the TABP

p instruction. If URS flag is “0,” the contents of the most

significant 1 bit of ROM code is not referred even when

executing the TABP p instruction. However, if URS flag is “1,”

the contents of the most significant 1 bit of ROM code is set to

flag CY when executing the TABP p instruction (Figure 4).

URS flag is “0” after system is released from reset and returned

from RAM back-up mode. It can be set to “1” with the URSC

instruction, but cannot be cleared to “0.”

(6) 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;

• performing a subroutine call, or

• executing the table reference instruction (TABP p).

Stack registers (SKs) are four identical registers, so that

subroutines can be nested up to 4 levels. However, one of

stack registers is used when executing a table reference

instruction. Accordingly, be careful not to over the stack. The

contents of registers SKs are destroyed when 4 levels are

exceeded.

The register SK nesting level is pointed automatically by 2-bit

stack pointer (SP).

Figure 5 shows the stack registers (SKs) structure.

Figure 6 shows the example of operation at subroutine call.

(7) Skip flag

Skip flag controls skip decision for the conditional skip

instructions and continuous described skip instructions.

Note : The 4282 Group just invalidates the next instruction

when a skip is performed. The contents of program

counter is not increased by 2. Accordingly, the number

of cycles does not change even if skip is not performed.

However, the cycle count becomes “1” if the TABP p,

RT, or RTS instruction is skipped.

SK0

SK1

SK2

SK3

(SP) = 0

(SP) = 1

(SP) = 2

(SP) = 3

Program counter (PC)

Executing RT

instruction

Executing BM

instruction

Stack pointer (SP) points “3” 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 four

stack registers are used ((SP) = 3), (SP) = 0

and the contents of SK0 is destroyed.

Returning to the BM instruction execution

address with the RT instruction, and the BM

instruction is equivalent to 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)

← 3

Note:

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