4507 Datasheet, PDF (13/115 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4507 Group
(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.
Program counter (PC)
Executing BM
instruction
Executing RT
instruction
SK0
(SP) = 0
SK1
(SP) = 1
SK2
(SP) = 2
SK3
(SP) = 3
SK4
(SP) = 4
SK5
(SP) = 5
SK6
(SP) = 6
SK7
(SP) = 7
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.
Fig. 5 Stack registers (SKs) structure
(SP) ← 0
(SK0) ← 000116
(PC) ← SUB1
Main program
Address
000016 NOP
000116 BM SUB1
000216 NOP
Subroutine
SUB1 :
NOP
·
·
·
RT
(PC) ← (SK0)
(SP) ← 7
Note : Returning to the BM instruction execution
address with the RT instruction, and the BM
instruction becomes the NOP instruction.
Fig. 6 Example of operation at subroutine call
Rev.3.01 2005.02.04 page 13 of 111
REJ03B0107-0301