1. Intel’s 8086 instruction-set
A look at the main categories of processor instructions for the earliest Intel x86 CPU

2. Data-Transfer
These instructions simply copy some data from one place in the system to another
From one register to another register
From a memory location to a register
From a register to a memory location
From within an instruction to a register
From within an instruction to a memory cell

3. ‘mov’ It’s the most frequently-used instruction immediate data
general register
general register
or memory
segment register

4. Some other data-transfers
Exchange (‘xchg’)
Push/Pop
Pushf/Popf
In/Out
Load pointer (‘lds’, ‘les’, ‘lfs’, lgs’, ‘lss’)
Note: A distinctive feature data-transfers is that FLAGS are unmodified (except ‘popf’)

5. Arithmetic Instructions
Addition: ‘add’ and ‘adc’
Subtraction: ‘sub’ and ‘sbb’
Multiplication: ‘mul’ and ‘imul’
Division: ‘div’ and ‘idiv’
Increment/Decrement: ‘inc’ and ‘dec’
Comparison/Negation: ‘cmp’ and ‘neg’
All these instructions do modify FLAGS

6. Boolean Logic Instructions
Bitwise AND: ‘and’
Bitwise OR: ‘or’
Bitwise XOR: ‘xor’
Bitwise TEST: ‘test’
Bitwise complement: ‘not’
These instructions also modify FLAGS

7. Shift/Rotate Instructions
Left-shifts: ‘shl’ and ‘sal’
Right-shifts: ‘shr’ and ‘sar’
Left-rotations: ‘rol’ and ‘rcl’
Right-rotations: ‘ror’ and ‘rcr’
These instructions affect the Carry-Flag

8. Control Transfers
These instructions modify register RIP so as to alter the normal fetch-execute cycle
Unconditional Jumps: ‘jmp’ and ‘ljmp’
Subroutine Calls: ‘call’ and ‘lcall’
Subroutine Returns: ‘ret’ and ‘lret’
Conditional Jumps: ‘jz’, ‘jnz’, ‘jc’, ‘jnc’, etc
These transfers do not modify the FLAGS

9. String Manipulations
This unusual group of complex instructions is for efficient word-processing operations
Move string: ‘movs’
Compare string: ‘cmps’
Scan string: ‘scas’
Store string: ‘stos’
Load string: ‘lods’
Repeat prefixes: ‘rep’, ‘repe’ and ‘repne’

10. Processor Control
This group of instructions is for specialized systems programming situations
Halt the fetch-ececute cycle: ‘hlt’
Wait for coprocessor to finish: ‘wait’
Lock the system bus temporarily: ‘lock’
Adjust the Direction Flag: ‘cld’ or ‘std’
Adjust the Interrupt Flag: ‘cli’ or ‘sti’
Adjust the Carry Flag: ‘cmc’, ‘clc’ or stc’

11. Special-purpose instruction
initializations
the loop body
This two-step construct
occurs so often within
ordinary programs that
a special instruction is
available to do both in
a single instruction
decrement RCX
RCX == 0? no
yes

12. Stack operation: ‘push’
register
data
64-bits
64-bits
data
data
data
data
data
data
SS:RSP
unused
data
SS:RSP
unused
unused
unused
unused
before ‘push’’
after ‘push’
The ‘push’ instruction, applied to a register, performs a two-step operation:
first, the value in the stack-pointer register is decreased by 8, and then the
value in the register operand is copied into the stack-location at SS:RSP

13. Stack operation: ‘pop’
register
64-bits
64-bits
data
data
data
data
data
data
SS:RSP
data
unused
SS:RSP
unused
unused
unused
unused
before ‘pop’’
after ‘pop’
The ‘pop’ instruction, applied to a register, performs a two-step operation:
first, the value in the stack-location at SS:RSP is copied into that register,
and afterward the value in the stack-pointer register is increased by 8.

14. An application
We can apply these stack-operations to a string of characters in a memory-buffer, so as to reverse the order of those characters
stack A B C D
RSP
buffer
buffer
A B C D
D C B A
pop
pop
pop
pop
push
push
push
push

15. ‘reverse.s’
We’ve constructed a demo-program that reverses the order of characters which a user types in
It uses the ‘push’ and ‘pop’ operations, as well as the special x86 ‘loop’ instruction
It also employs the ‘read’ system-call to get a string of characters typed by a user

16. The program’s flow Prompt the user for input INPUT
Accept the user’s response
Loop to ‘push’ each buffer character onto stack
PROCESS
Loop to ‘pop’ each character to output buffer
Show rearranged characters in output buffer
OUTPUT

17. Loops are ‘consecutive’

18. Loops are ‘nested’

19. In-class exercise
Can you modify the ‘reverse.s’ program so all words remain in the same order, but the order of the letters within each word do get reversed?
HINT: You will need a loop-within-a-loop
This is our moment, this is our time.
INPUT
sihT si rou themom, siht si rou emit.
OUTPUT