Department of Control and Systems Engineering Lecture 2 – Page
8
of 10 . Third Year - Microprocessors
By Mr.WaleedFawwaz
9. The stack
The stack is implemented in the memory and it is used for temporary storage of information such as data and addresses. The stack is 64Kbytes long and is organized from
a software point of view as 32Kwords see Fig 10.
• SS register points to the lowest address word in the stack • SP and BP points to the address within stack
• Data transferred to and from the stack are word-wide, not byte-wide.
• The first address in the Stack segment SS : 0000 is called End of Stack. • The last address in the Stack segment SS : FFFE is called Bottom of Stack.
• The address SS:SP is called Top of Stack.
• POP instruction is used to read wordfrom the stack. • PUSH instruction is used to write word to the stack.
• When a word is to be pushed onto the top of the stack: o
the value of SP is first automatically decremented by two o
and then the contents of the register written into the stack. • When a word is to be popped from the top of the stack the
o the contents are first moved out the stack to the specific register
o then the value of SP is first automatically incremented by two.
Fig 10: Stack segment of memory Example 5: let AX=1234H ,SS=0105H and SP=0006H. Fig 11 shows the state of stack prior
and after the execution of next program instructions: PUSH AX
POP BX POP AX
Department of Control and Systems Engineering Lecture 2 – Page
9
of 10 . Third Year - Microprocessors
By Mr.WaleedFawwaz
Fig 11PUSH and POP instruction
10. Input and Output address space The 8086 has separate memory and inputoutput IO address spaces. The IO address
space is the place where IOinterfaces, such as printer and monitor ports, are implemented. Notice that this address range is form 0000H to FFFFH. This represents just
64Kbyte addresses; therefore only 16 bits of address are needed to address IOspace.
01054 01055
01056 01057
01050 01051
01052 01053
01058 01059
0105A 0105B
SS SP
BX
0105 0006
5D00 55
1F DF
DD
02 00
C0 52
90 68
A2 55
01054 01055
01056 01057
01050 01051
01052 01053
01058 01059
0105A 0105B
SS SP
BX
0105
0004
5D00
34 12
DF DD
02 00
C0 52
90 68
A2 55
01054 01055
01056 01057
01050 01051
01052 01053
01058 01059
0105A 0105B
SS SP
BX
0105
0006 1234
34 12
DF DD
02 00
C0 52
90 68
A2 55
01054 01055
01056 01057
01050 01051
01052 01053
01058 01059
0105A 0105B
SS SP
BX
0105
0008
1234 34
12 DF
DD
02 00
C0 52
90 68
A2 55
a Initial state b After execution of PUSH AX
d After execution of POP AX c After execution of POP BX
AX 1234
AX 1234
AX 1234
AX DDDF
Department of Control and Systems Engineering Lecture 2 – Page
10
of 10 . Third Year - Microprocessors
By Mr.WaleedFawwaz
Problems 1.
What are the length of the 8086’s address bus and data bus? 2.
How large is the instruction queue of the 8086? 3.
List the elements of the execution unit. 4.
What is the maximum amount of memory that can be active at a given time in the 8086?
5. Which part of the 8086’s memory address space can be used to store the
instruction of a program? 6.
Name two dedicated operations assigned to the CX register. 7.
Calculate the value of each of the physical addresses that follows. Assume all numbers are hexadecimal numbers.
a A000 : ? =A0123
b ? : 14DA =235DA
c D765 : ? =DABC0
d ? : CD21 =32D21
8. If the current values in the code segment register and the instruction pointer are
0200
16
AND 01AC
16
9. If the current values in the stack segment register and stack pointer are C000
, respectively, what physical address is used in the next instruction fetch?.
16
and FF00
16
, respectively, what is the address of the current top of the stack?
Department of Control and Systems Engineering Lecture 3 – Page
1
of 8 . Third Year - Microprocessors
By Mr.WaleedFawwaz
Lecture 3- Addressing MODES
1. Introduction to assembly language programming
