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CHAPTER 6 STACK
The stack is a register used to save information such as the program return address and the contents of the system
register during execution of subroutine calls, interrupts and similar operations.
6.1 STACK CONFIGURATION
Figure 6-1 shows the stack configuration.
The stack consists of the following parts: one 3-bit binary counter stack pointer, five 10-bit address stack registers,
and one 5-bit interrupt stack registers.
Figure 6-1. Stack Configuration
b
9
Address Stack Register
(ASR)
b
8
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
b
10
Address stack register 0
Address stack register 1
Address stack register 2
Address stack register 3
Address stack register 4
0H
1H
2H
3H
4H
b
2
SPb
2
b
1
SPb
1
b
0
SPb
0
Stack Pointer
(SP)
BCDSK
Interrupt Stack Register
(INTSK)
CMPSK
CYSK
ZSK
IXESK
0H
6.2 FUNCTIONS OF THE STACK
The stack is used to save the return address during execution of subroutine calls and table reference instructions.
When an interrupt occurs, the program return address and the program status word (PSWORD) are automatically
saved in the stack.
Remark
All the 5 bits of PSWORD are automatically cleared to zero after being saved in the interrupt stack
register.
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