ZiLOG Z8 Technical Manual

Address Spaces
3.6 CPU STACKS
Stack operations can occur in either the register
file or data memory. Under software control,
Port a and 1 Mode register (R258) selects stack
location.
The register pair R254 and R255 forms the 16-bit
Stack Pointer (SP) which is used for all stack
operations. The stack address is stored with the
MSB in R254 and LSB in R255 (Figure 3-7).
3-6
R255
lOWER BYTE STACK POINTER lOW
R254
.... ___ U_P_P_E_R_BY_T_E ___ ... I STACK POINTER HIGH
Figure 3-7. Stack Pointer
•
•
•
PCl
TOP OF ----+- PCH
STACK
STACK CONTENTS
AFTER A CAll
INSTRUCTION
Figure 3-B.
The stack address is decremented prior to a Push
operation and incremented after a Pop operation.
The stack address always points to the data stored
on the top-of-stack. The Z8 stack is a return
stack for Call instructions and interrupts as well
as a data stack. During a Call instruction, the
contents of the PC are saved on the stack. The PC
is restored during a Return instruction. Inter-
rupts cause the contents of the PC and Flag regis-
ter to be saved on the stack. The IRET instruc-
tion restores them (Figure 3-8).
When the Z8 is configured for an internal stack
(i.e., using the register file), register R255
serves as the Stack Pointer. The value in R254 is
ignored and can be used as a general-purpose
register. However, an overflow or underflow can
occur
when stack address is incremented or
decremented during normal stack operations.
TOP OF---.
STACK
Stack Operations
•
•
0
PCl
PCH
FLAGS
STACK CONTENTS
AFTER AN
INTERRUPT
CYCLE
3047-049,3047-074

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English