Chapter 6
Program Control Instruction
Introduction
program control instruction :
direct the flow of a program, allow the flow to change
jumps, calls, returns, interrupts, machine control
instructions
change in flow :
CMP, TEST followed by conditional jump
relational assembly language statements :
.IF, .ELSE, .ELSEIF, .WHILE, .ENDW, .REPEAT,
.
UNTIL
MASM, TASM Ver.6X ~
allow to develop control flow portions of program with
C/C++ language efficiency
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Instructions
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6-1 The Jump Group
JMP(jump) : allow to skip sections of a program and
blanch to any part of memory for next instruction
unconditional jump, conditional jump
three type unconditional jump : Fig. 6-1
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Unconditional Jump(JMP)
intrasegment jump : short, near jump
Short jump(2-byte): 1 byte disp.(within +127~-128 byte)
Near jump(3-byte) : 2 byte disp.(within 32K bytes or
anywhere in current code segment)
segments : cyclic in nature
intersegment, far jump(5-byte) :
any memory location within the real memory system
80386~ (in protected mode)
Near(5-byte) : 4 byte displacement(within 2G bytes)
Far(7-byte) : 4 byte(EIP), 2 byte(CS)
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Instructions
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Short Jump
short jump : relative jump
distance or displacement : follow the opcode
one-byte signed number(+127~-128) :
sign-extended and added to IP/EIP
to generate the jump address within current code segment
EX. 6-1 :
label : symbolic name for memory address
SHORT directive : force a short jump
most assembler : choose best form of jump instruction
JMP START : assemble as a short jump
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Instructions
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Short Jump
1st jump : 0020H – 0009H = 0017(disp. = 17H)
2nd jump : 0002H – 0024H = FFDEH(disp. = DEH)
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Fig. 6-2
Fig. 6-2
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Near, Far Jump
near jump : relocatable because relative jump
signed displacement : added to IP/EIP to generate
the jump address
2 byte : 32K bytes in current code segment
4-byte(386~ in protected mode) : 2G bytes
far jump : 5(7, 80386~) byte instruction
new offset address(IP/EIP) : byte 2,3(2~5)
new segment address(CS) : byte 4,5(6,7)
80286~ in protected mode : CS access a descriptor
that contain base address of far jump segment
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Fig. 6-3
Fig. 6-3
Ch.6 Program Control
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EX. 6-2 : Near Jump
E9 0200 R JMP NEXT : only list file
R : denote a relocatable jump address of 0200H
actual machine code : E9 F6 01
0200H - 000AH = 01F6H
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Fig. 6-4
Fig. 6-4
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EX. 6-3
far jump : FAR PTR directive, far label
far label : external to current code segment
EXTRN UP:FAR directive
a global label as a double colon(LABEL::)
----E : external. filled in by linker when links program files
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Indirect Jump
jump with 16-, 32-bit reg. operand : indirect jump
contents of reg. : transferred directly into IP/EIP
JMP AX : IP ← AX, JMP EAX : EIP ← EAX
EX. 6-4 : how JMP AX access jump table
read a key, converted ASCII to binary, doubled
jump table : 16-bit offset address
Indirect Jumps using Index : double-indirect jump
[ ] form of addressing to directly access jump table
near jump JMP TABLE[SI] : IP ← [SI+TABLE]
far jump JMP FAR PTR [SI], JMP TABLE [SI] with
TABLE data defined DD directive
Ch.6 Program Control
Instructions
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EX. 6-4
EX. 6-4
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EX. 6-5
EX. 6-5
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Conditional Jumps
conditional jump : short jump
~ 80286(short jump) : +127 ~ -128
80386 ~(short, near jump) : 1, 4 bytes
test one flag bit or some more : S, Z, C, P, O
if condition under test is true : branch to the label
if condition is false : next sequential instruction
relative magnitude comparisons :
require more complicated conditional jump instructions
that test more than one flag bit
Table 6-1 : conditional jump instructions
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Instructions
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Table 6-1
Table 6-1
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Fig. 6-5
Fig. 6-5 : order of signed, unsigned 8-bit no.s
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Conditional Jumps
unsigned : FFH is above 00H, above, below, equal
signed : FFH less than 00H, greater, less, zero
alternate form :
JE = JZ
JA(if above) = JNBE(if not below or equal)
JCXZ(jump if CX = 0), JECXZ(jump if ECX=0)
if CX/ECX = 0 : jump occur
if CX/ECX <> 0 : no jump occur
EX. 6-6 : search table for 0AH using SANSB, JCXZ
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EX. 6-6
EX. 6-6
Ch.6 Program Control
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Conditional Set Instructions
conditional set instructions :
80386~
set a byte to either a 01H or clear a byte to 00H
useful where a condition must be tested at a point much
later in the program
SETNC MEM :
places a 01H into memory location MEM if carry is
cleared and
a 00H into MEM if carry is set
Table 6-2 :
Ch.6 Program Control
Instructions
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Table 6-2
Table 6-2
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LOOP, Conditional LOOP
LOOP : combination of decrement CX and JNZ
~ 80286 : DEC CX ; if CX <> 0, jump to label
CX = 0, execute next sequential instruction
80386 ~ : CX/ECX depending on instruction mode
if
LOOPE(loop while equal, LOOPZ) :
jump if CX <> 0 while equal condition exist
exit the loop if CX = 0 or condition is not equal
LOOPNE(loop while not equal, LOOPNZ) :
jump if CX <> 0 while not-equal condition exist
exit the loop if CX = 0 or condition is equal
LOOPEW/LOOPED,LOOPNEW/LOOPNED:override mode
Ch.6 Program Control
Instructions
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EX. 6-7
EX. 6-7 :
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6-2 Controlling the Flow of an Assembly
Language Program
relational statements
.IF, .ELSE, .ELSEIF, ENDIF, .REPEAT.UNTIL, .WHILE-.ENDW :
easier to control the flow than conditional jump
EX. 6-8 : testing system for version of DOS
DOS INT 21H, function no. 30H : read DOS ver.
(a) : source program, (b) fully expended assembled
* : assembler-generated and -inserted statements
&& : logical AND
Table 6-3 : relational operator
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Table 6-3
Table 6-3
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EX. 6-10
EX. 6-10 : read a key, convert to hexadecimal
`a`(61H), `A`(41H) : 61H(41H)-57H(37H)=0AH
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DO-WHILE Loops
.WHILE statement : used with a condition to
begin the loop
EX. 6-11 : read a key, store into array called BUF
until enter key(0DH) is typed
DOS 21H, fn no. 09H
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Instructions
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EX. 6-11
EX. 6-11
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REPEAT-UNTIL Loops
.REPEAT : defined start of loop
.UNTIL : defined end of loop, contained condition
EX. 6-14 : EX. 6-11,12
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EX. 6-14
EX. 6-14
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6-3 Procedures
Procedure :
a group of instructions that usually performs one task
a reusable section of the software that is stored in
memory once, but used as often as necessary
advantage :
save memory space
make it easier to develop software
disadvantage :
take the computer a small amount of time to link to
procedure and return from it
CALL/RET : link to/return from the procedure
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Instructions
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Procedure
CALL : push the address of instruction following
CALL(return address) on stack
RET : remove an address from stack so the
program return to instruction following CALL
specific rules for storing procedure
begin with PROC, end with ENDP directive
each directive : appear with name of procedure
PROC : followed by type of procedure : NEAR,FAR
type :can be followed by the USES statement
USES statement : allow any no. of reg. to be
automatically pushed and popped within procedure
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EX. 6-16
EX. 6-16
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CALL
near return(C3H) : remove 16-bit no. from stack,
place it into IP to return from procedure in current
segment
far return(CBH) : remove 32-bit no. from stack,
place it into both IP, CS to return from procedure
to any memory location
far procedure : global, used by all software
near procedure : local, used by a given task
CALL : differ from jump instruction
because a CALL save a return address on stack
Ch.6 Program Control
Instructions
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Near CALL
near CALL : 3(5, 80386~ in protected mode)-byte
instruction
1st byte : opcode
2nd, 3rd byte : displacement(distance) of 32K
2nd~5th byte : 32-bit displacement of 2G bytes
near CALL execute :
push offset address of next instruction(IP/EIP) on stack
add displacement from byte 2,3(2~5) to IP/EIP to
transfer control to the procedure
CALLN(near CALL)
Fig. 6-6 :
Ch.6 Program Control
Instructions
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Fig. 6-6
Fig. 6-6
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Far CALL
far CALL : 5(7, 80386~ in protected mode)-byte
instruction
1st byte : opcode
2nd 3rd byte : new IP, 4th 5th byte : new CS
2nd~5th byte : new EIP, 6th 7th byte : new CS
far CALL execute :
push IP/EIP, CS on stack
place byte 2,3(2~5) to IP/EIP and byte 4,5(6,7) to CS to
call a procedure located anywhere in memory system
CALLF(far CALL)
Fig. 6-7
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Instructions
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Fig. 6-7
Fig. 6-7
Ch.6 Program Control
Instructions
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CALLs with Register, indirect address
CALL with register operand :
like jump, also contain a register operand
CALL BX : push IP, jump to offset address located
in BX(IP ← BX) in current code segment
CALL with indirect memory address :
useful whenever different subroutines need to be chosen
CALL : also reference far pointers
CALL FAR PTR [SI] or CALL TABLE[SI]
data in table : defined as doubleword data with DD
retrieve a 32-bit address from data segment addressed
by SI, use it as address of a far procedure
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Instructions
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EX. 6-17
EX. 6-17 : display ‘OK’
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RET
RET : real mode(80386~ in protected mode)
near RET: remove 16-bit(32-bit), place it into IP/EIT
far : remove 32-bit(6 bytes), place it into IP/EIP, CS
near, far return : defined in procedure’s PROC
other form : RET n
n(bytes) : add n to contents of SP after return address is
removed from stack
push passing parameters on stack before calling procedure
if these parameters are to be discarded upon return,
RET contains a no. that represents the no. of bytes pushed
to stack as parameters
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Fig. 6-8 : near return
Fig 6-8
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EX. 6-19
EX. 6-19
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RET
RETN : CALLN
RETF : CALLF
passing parameters to a procedure :
1. to use one of the CPU register : MOV CX, TI
2. to use a memory location : MOV TEMP, TI
3. to pass the address of memory location :
MOV SI, OFFSET TI
4. to pass the parameters on the stack : (EX. 6-19)
MOV DX, TI, PUSH DX
5. to use stack frame : ENTER, LEAVE(p.211)
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6-4 Introduction to Interrupt
1. hardware-generated CALL : external interrupt
externally derived from a hardware signal
2. software-generated CALL : internal, exception
internally derived from the execution of an instruction or
by some other internal event()
interrupt : interrupts the program by calling an interrupt
service procedure or interrupt handler
interrupt vector :
in real : 4-byte no. stored in 1st 1024 bytes(~0003FFH)
256(00H~FFH)  4byte = 1024byte
protected : replaced by interrupt descriptor table
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Instructions
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Interrupt Vectors
Table 6-4 : 256 different interrupt vectors in real
each contain address of an interrupt service procedure
for IP, CS
Intel reserve the 1st 32 interrupt vector(~1FH) : for
present , future µ
remaining : available for user
some of reserved : for error that occur during execution
of software, such as divide error interrupt
some of reserved : for coprocessor
others : occur for normal events in the system
vectors 1-6,7,9,17 : function in real, protected mode
remaining : only in protected mode
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Table 6-4
Table 6-4
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Interrupt Instructions
software interrupt instruction : special type of CALL
INT, INTO, INT3
each of these instruction :
1. in real, fetches vector from interrupt vector table
1. in protected, fetches an interrupt descriptor from
interrupt descriptor table
2. calls the interrupt service procedure
interrupt call : similar to far CALL instruction
because placed return address(IP/EIP, CS) on stack
different : pushed flags, then pushed return address
fetched new value IP/EIP, CS from vector
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Instructions
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INTs
INT n : 256 different software interrupt instruction
type no. n : 0 ~ 255(00H ~ FFH)
INT 100 : uses interrupt vector no. 100(64H)
memory address in IVT : 190H~193H
0110 0100(64H) → shift left 2 → 01 1001 0000(190H)
address of interrupt vector in real :
multiplying type no. times 4(each vector : 4 bytes)
address of interrupt descriptor in protected :
multiplying type no. times 8(each descriptor : 8 byte)
INT : 2-byte long(1st:opcode, 2nd:vector type no.)
INT 3: 1-byte special software interrupt for breakpoints
Ch.6 Program Control
Instructions
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INTs
software interrupt instruction execute :
(1) push flags (2) clear T, I flag bits (3) push CS (4)
fetch new value for CS from interrupt vector (5) push
IP/EIP (6) fetch new value for IP/EIP from vector (7)
jump new location addressed by CS, IP/EIP
INT : perform PUSHF, followed by far CALL
I flag : control external hardware interrupt input pin,
INTR(maskable interrupt request)
I = 0 : disable the INTR pin
T flag : trap, single step interrupt
T = 0 : disable the single step interrupt
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Instructions
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INTs
software interrupt :
most commonly used to call system procedure
because address of system function need not be known
system procedure(function) :
common to all system, application software
software interrupt :
often control printers, video displays, disk drives
relieving the program from remembering address of
system call
INT(2-byte long) : used to replace a far CALL(5-byte)
Ch.6 Program Control
Instructions
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IRET/IRETD
IRET : used only with software or hardware interrupt
service procedures
IRET : perform POPF, followed by far RET
(1) pop stack data back into IP/EIP (2) pop into CS (3)
pop into flag register
IRETD : 80386~ in protected mode
differ from IRET, pop 32-bit EIP from stack
RET 3 : 1-byte instruction
special software interrupt designed to function breakpoint
interrupt or break the flow of software
breakpoint : help to debug faulty software
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Instructions
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Interrupt
INTO(interrupt on overflow) :
conditional software interrupt that test overflow flag(O)
if O = 1 : interrupt vector no. 4 occur
appear in software that add, subtract signed binary no.
interrupt service procedure :
same as far procedure
different : ends with IRET, saved and restored flag
register
interrupt control
STI(set interrupt flag) : I ← 1, enable INTR pin
CLI(clear interrupt flag) : I ← 0, disable INTR pin
Ch.6 Program Control
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EX. 6-20
EX. 6-20 : add DI, SI, BP, BX, save sum in AX
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Interrupts in the Personal Computer
Interrupts found in the personal computer o
nly contained Intel-specified interrupts 0–4.
Access to protected mode interrupt structur
e in use by Windows is accomplished throu
gh kernel functions Microsoft provides.
and cannot be directly addressed
Protected mode interrupts use an interrupt
descriptor table.
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Figure 6–9 Interrupts in a typical personal computer
.
Figure 6–9 Interrupts in a typical personal computer.
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6-5 Machine Control and Miscellaneous Instructions
carry flag(C) : carry(addition), borrow(subtraction)
STI(set carry)
CLC(clear carry)
CMC(complement carry)
indicate error in procedure(ex. reads data from a disk
memory file) : successful or file-not-found error
WAIT :
monitor BUSY’(286,386)/TEST’(8086/88) input pin
BUSY’ pin : connected to BUSY’ pin of coprocessor
BUSY’ = 1 : nothing happen, next instruction execute
BUSY’ = 0 : µ wait for BUSY’ pin return to logic 1, µ
wait until coprocessor finishes a task
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HLT, NOP
HLT(halt) :
stop the execution of soft ware
three ways to exit a halt : by an interrupt, by hardware
reset, during a DMA operation
normally appears in a program to wait for an interrupt
NOP(no operation)
take a short time to execute no operation
also used in time delays to waste time : not very
accurate(cache, pipeline in modern µ)
often used to pad software with space for future
machine language
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Instructions
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LOCK prefix, ESC
LOCK prefix :
cause LOCK’ output pin to activate(logic 0) for
duration of a locked instruction
LOCK’ pin : often disables external bus masters or
other system components
ex. : LOCK:MOV AL,[SI]
ESC(escape) :
pass information to numeric coprocessor
6 bits of the ESC instruction : provide the opcode to
coprocessor and begin executing an instruction
ESC opcode : never appear
code prefixed in coprocessor instruction(FLD,FST..)
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BOUND
BOUND : 80186~
compare contents of any 16- or 32-bit reg. against contents
of two word or doubleword of memory: a lower and an
upper boundary
if value of reg. is not within the boundary : type 5 interrupt
if within the boundary : next instruction execute
ex. BOUND SI, DATA
word-sized location DATA : lower boundary
word-sized location DATA+2 : upper boundary
if SI  WORD PTR [DATA] or WORD PTR [DATA+2] 
SI : type 5 interrupt
return address : point to BOUND instruction
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Instructions
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ENTER and LEAVE
ENTER(two operand) : create stack frame
by pushing BP onto stack and
loading BP with uppermost address of stack frame
stack frame variables : accessed through BP
1st operand : no of byte to reserve for variables on stack
frame, 2nd : level of procedure
stack frame : mechanism used to pass parameters to a
procedure through the stack memory
LEAVE : reverse ENTER
by reloading SP, BP with their prior values
ENTER 8,0 : reserve 8 bytes for stack frame(Fig6-9)
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Fig. 6-9
Fig. 6-9
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EX. 6-21
EX. 6-21 : create a stack frame so that two 16-bit
parameters are passed to a system level procedure
64
EX. 6-21
EX. 6-21 :
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