Flow control instructions

Lecture 1
Instruction set of 8086
Лектор: Люличева И.А.
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Content
80286 MP block-diagram
Universal microprocessors
instruction groups
Examples of programs
80286 MP block-diagram
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80286 MP
The second generation of x86 16-bit
processors, Intel 80286, was released in
1982. The major new feature of the
80286 microprocessor was protected
mode. Instruction set was changed by
adding some new instructions to the
existing six groups.
Universal microprocessors
instruction groups
Instruction set of MP x86 consist of 6 groups:
 1. Data transfer instructions.
 2. Arithmetical instructions.
 3. Logical operations and shift instructions.
 4. Flow control instructions.
 5. Chains (arrays) instructions.
 6. Microprocessor control instructions.
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Basics of instruction set of MP
or MC
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3 types of
instruction
format
http://www.cjump.com/
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Flow control instructions
Segmentation leads to two types of the flow
control:
 Inside the segment - only IP changes. Type NEAR, or short addressing.
 Outside the segment. Type - FAR - both IP and
CS change (segment: bias) ES:DI
 Segment registers are
CS, DS, ES, SS
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Flow control instructions
May be divided on 5 subgroups:
 Unconditional jumps,
 Conditional branches,
 Subroutines call and return,
 cycles
 Interrupts instructions
Let us look at each groups .
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Flow control instructions
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Unconditional JMPs may be short or long and
direct/indirect.
JMP dispL16- short jump
JMP mem/reg - near indirect jump
JMP addr32 - far jump
JMP mem - far indirect jump
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Flow control instructions
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19 conditional branches
All of them are short and may be executed ander some
condition:
Mnemonics:
1) for unsigned data JA/JNBE
- if more (>);
JAE/JNB/JNC - >=
JB/JNAE/JC - <;
JBE/JNA
- <=.
2) For signed data:
JG/JNLE - >; JGE/JNL - >=;
JL/JNGE - <; JLE/JNG - <=;
JNS
- >0; JS
- <0.
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Flow control instructions
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3) Conditional jumps: JE/JZ — jump if equal
(if zero); JNE/JNZ — if not zero ;
JNO/JNC - jump if not carry;
JO/JC- jump on carry.
Additional instr:
JCXZ - jump ix CX=0;
JNP/JPO - jump if odd (not parity);
JP/JPE - if even (if parity) .
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Flow control instructions
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3 instr for cycles - use CX as a counter. Only short
jumps on -128 - +127 bytes from current point.
LOOP make CX-- and continue the cycle until
CX=0
For example
@m1:
…
Loop @m1
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Flow control instructions
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Short delay subroutin
MOV CX, 1000
M1: DEC CX
JNZ M1
You my write the same in a short way, using
loop instruction:
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MOV CX, 1000
M1: LOOP M1
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Flow control instructions
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For work with subroutines
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Call subroutine: Call @Pr1
….
@Pr1:
Push ax
Add ax, bx
Mov ES:[di], ax
Pop ax
Ret
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6. MP control instructions
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Control MP functioning.
For 8086 only work with flags and
synhroniz.(NOP, wait etc).
In 80286, 80386 ++ - additional 10 - 20
mnemonics.
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Work with chains
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Chain is a consequence of chained bytes or
words, которые находятся в смежных
ячейках памяти.
MP 8086 has 5 chains instructions
They may work with the special prefixes
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Work with chains
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Simple pref is REP, wich leads to the repetition of
action on the next chain element. Повторение
рассчитано на максимальную длину
цепочек 64К и выполняется значительно
быстрее цикла LOOP.
Цепочечные команды могут иметь только один
операнд-источник, или операнд-получатель или
оба операнда одновременно. В качестве адреса
операнда-источника (SRC) всегда используется
регистровая пара DS:SI.
Операнд-получатель – всегда пара ES:DI.
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Work with chains
Instruction MOVS sends one byte or
word from the chain, addressed by
DS:SI, into the chain, addressed by
ES:DI.
 Format: MOWSB or MOVSW
 Works with the REP prefix
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Work with chains
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Compare bytes or words in a chain
with AL or AX CMPS
Formate: CMPSB or CMPSW
Works with REPE or REPNE prefix
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Work with chains
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Instructions STOSB/W save AL or AX into
a chain, addressed by ES:DI.
Format STOSB, STOSW
Below are 2 examples of long description of
these instr.
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Instruction STOSB
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Store byte in AL into ES:[DI]. Update DI.
Algorithm:
ES:[DI] = AL
if DF = 0 then
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DI = DI + 1
else
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DI = DI - 1
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Instruction STOSW
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Store word in AX into ES:[DI]. Update DI.
Algorithm:
ES:[DI] = AX
if DF = 0 then
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DI = DI + 2
else
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DI = DI - 2
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Work with chains
Example of using STOSB/W for clear screen.
Block diagram
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Work with chains
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Example of program.
MOV AX, 0B800H { Do not repeat
MOV ES, AX
{ futher! }
XOR DI,DI
MOV CX, 80*25
XOR AX,AX
REP STOSW
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Examples of programs
Block-diagr of using STOSB/W for filling 10th
line of Pc screen by number 3.
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Examples of programs
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Example of using STOSB/W for filling 10th line
of Pc screen by number 3.
MOV DI, 160*10 (1 -2 bytes!)
MOV CX, 80
MOV AX, 1E33Н (33Н – ASCII code for 3)
REP STOSW
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Examples of programs
Block-diagr for filling the colomn.
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Self-control questions
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Groups of instr 8086
Far and long jumps.
Поясните особенности команд работы с цепочками
и сегментных регистров.
Приведите примеры использования команд
работы с цепочками.
Приведите пример программы циклического
опроса 20 портов и укажите использованные виды
адресации
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