1. Program Control Instructions
A Course in Microprocessor
Electrical Engineering Dept.
University of Indonesia

2. The Jump Group Unconditional Jump (fig.6.1) Short Jump
Jump (JMP) allows the programmer to skip sections of a program and branch to any part of the memory for the next instruction
A Conditional Jump allows the programmer to make decisions based upon numerical test
Unconditional Jump (fig.6.1)
Short Jump
a 2-byte instruction that allows jumps or branches to memory locations within +127 and -128 bytes from the address following the jump
is called relative jump

3. The Jump Group (cont’d)
the jump address is not stored with the opcode but a distance or displacement follows the opcode
See fig.6.2 and study example 6.1
Near Jump
It passes control to an instruction in the current code segment located within 32K bytes from the near jump
The near jump is relocatable known as relative jump
See fig.6.3 and study example 6.2
Far Jump
It obtains new segment and offset address to accomplish the jump (fig.6.4 and example 6.3)

4. The Jump Group (cont’d)
Jumps with Register Operands
known as indirect jump
the address of the jump is in the register specified by the jump instruction (i.e., the contents of the register are transferred directly into the instruction pointer)
Study example 6.4
Indirect Jump Using an Index
It uses the [] form of addressing to directly access the jump table
The JMP Table [SI] instruction (example 6.5) points to a jump address stored at the code segment offset loction addressed by SI

5. The Jump Group (cont’d)
Conditional Jump and Conditional Sets
The conditional jump instructions test the following flag bits: sign (S), zero (Z), carry (C), parity (P), and overflow (O) --- see Table 6.1
if the condition under test is true, a branch to the label associated with the jump instruction occurs
Otherwise, the next sequential step in the program executes
The conditional jump instructions all test flag bits, except for the JCXZ (jump if CX=0) and JECXZ (study the example 6.6)
See also Table 6.2 for the conditional set instruction

6. The Jump Group (cont’d)
Loop
It is a combination of a decrement CX and JNZ conditional jump
Example 6.7 shows how to add data in a block of memory with data in another block of memory
Conditional Loops
LOOPE (loop while equal) jumps if CX <> 0 while an equal condition exists
LOOPNE (loop while not equal) jumps if CX <> 0 while a not-equal condition exists

7. Controlling the Flow of an Assembly Language Program
It is much easier to use the assembly language statements .IF., .ELSE., .ELSEIF., and .ENDIF. (study ex. 6.8, 6.9, 6.10 and Table 6.3)
DO-WHILE Loops
Pair: .WHILE and .ENDW
Study example 6.11, 6.12, and 6.13
REPEAT-UNTIL Loops
Pair: .REPEAT and .UNTIL
Study Example 6.14 and 6.15

8. Procedures CALL CALLs with Register Operands
A procedure is a group of instructions that usually performs one task
reusable - takes small amount of time
use stack - can be NEAR or FAR
Pair: PROC - ENDP (study example 6.16)
CALL
It transfers the flow of the program to the procedure
CALLs with Register Operands
E.g., CALL BX (example 6.17)

9. Procedures (cont’d) CALLs with Indirect Memory Address RET
useful whenever different subroutines need to be chosen from a program (see Example 6.18)
RET
It removes either a 16-bit (or 32-bit) number from the stack and places it into IP (and CS)
the new location (IP and CS:IP) is the address of the next instruction that immediately follows the most recent CALL to a procedure (Fig. 6.8)
Study Example 6.19

10. Introduction to Interrupt
An Interrupt is either a hardware-generated CALL (externally derived from a hardware signal) or a software-generated CALL(internally derived of the execution of an instruction or by some other internal event)
Interrupt Vectors
An interrupt vector is a 4-byte number stored in the first 1,024 bytes of memory (in the real mode)
The vector table is replaced by an interrupt descriptor table that uses 8-byte descriptors to describe each of the interrupts
There are 256 different interrupt vectors; each vector contains an address of an interrupt service procedure

11. Introduction to Interrupt (cont’d)
Interrupt Instructions
INT, INTO, and INT 3
INTs
256 software interrupt (INT) available
Whenever a software interrupt executes, it:
pushes the flags onto the stack
clears the T and I flag bits
pushes CS onto the stack
fetches the new value for IP/EIP from the vector
jump to the new leocation (CS:IP/EIP)

12. Introduction to Interrupt (cont’d)
IRET/IRETD
Used only with software or hardware interrupt service procedure
The IRET instruction will:
pop stack data back into the IP
pop stack data back into CS
pop stack data back into the flag register
INT 3
A special software interrupt designed to be used as a breakpoint
It is common to insert an INT 3 instruction in software to interrupt or break the flow of the software

13. Introduction to Interrupt (cont’d)
INTO
Interrupt on overflow is a conditional software interrupt that tests the overflow flag (O)
if O = 0 the INTO instruction performs no operation
if O = 1 an INTO instruction executes
It appears in software that adds or subtracts signed binary numbers --> INTO detects the overflow condition
An Interrupt Service Procedure (Ex. 6.20)
The main difference between this procedure and a normal far procedure is that it ends with the IRET instruction instead of the RET instruction, and the contents of the flag register are saved on the stack

14. Introduction to Interrupt (cont’d)
Interrupt Control
The set interrupt flag instruction (STI) enables the INTR pin
The clear interrupt flag instruction (CLI) disables the INTR pin
Interrupts in the Personal Computer
See Table 6.5

15. Machine Control and Miscellanous Instructions
These instructions provide control of the carry bit, sample the BUSY/TEST pin, and perform various other functions
Controlling the Carry Flag Bit
There are three instructions that control the contents of the carry flag: STC (set carry), CLC (clear carry) and CMC (complement carry)
WAIT
The WAIT instruction monitors the hardware BUSY/TEST pins

16. Machine Control and Miscellanous Instructions (cont’d)
HLT
The halt instruction that stops the execution of software
Three ways of to exit a halt: by an interrupt, by a hardware reset, or during DMA operation
NOP
It performs no operation (useful for delaying the operation)

20. Example 6.1 Example 6.2 0000 33 DB XOR BX,BX
B START: MOV AX,1
C ADD AX,BX
EB JMP SHORT NEXT
B D NEXT : MOV BX,AX
E8 DE JMP START
Example 6.2
DB XOR BX,BX
B START: MOV AX,1
C ADD AX,BX
E R JMP NEXT
B D NEXT : MOV BX,AX
E R JMP START

21. Example 6.3 0000 33 DB XOR BX,BX 0002 B8 0001 START: MOV AX,1
EXTRN UP : FAR
DB XOR BX,BX
B START: MOV AX,1
C ADD AX,BX
E R JMP NEXT
B D NEXT : MOV BX,AX
E R JMP FAR PTR START
EA E JMP UP

22. Example 6.4 ;A program that reads 1, 2 or 3 from the keyboard
;A program that reads 1, 2 or 3 from the keyboard
;if 1, 2 or 3 is typed , a 1, 2 or 3 is displayed ;
.MODEL SMALL ;select small model
DATA start of data segment
R TABLE DW ONE ;define lookup table
R DW TWO
R DW THREE
CODE start code segment
STARTUP ;start of program
0017 TOP:
0017 B MOV AH,1 ;read key into AL
0019 CD INT 21H
001B 2C SUB AL,31H ;convert to biner
001D 72 F8 JB TOP ;if below ‘1’ typed
001F 3C CMP AL,2
F4 JA TOP ;if above ‘3’ typed
0023 B MOV AH,00H ;double to 0, 2 or 4
C0 ADD AX,AX
0027 BE 0000 R MOV SI,OFFSET TABLE ;address lookup table
002A 03 F0 ADD SI,AX ;form lookup address
002C 8B MOV AX,[SI] ;get ONE, TWO or THREE
002E FF E0 JMP AX ;jump address
0030 ONE:
0030 B MOV DL,’1’ ;load ‘1’ for display
0032 EB JMP BOT ;go display ‘1’
0034 TWO:
0034 B MOV DL,’2’ ;load ‘2’ for display
0036 EB JMP BOT ;go display ‘2’
0038 THREE:
0038 B MOV DL,’3’ ;load ‘3’ for display
003A BOT:
003A B MOV AH,2 ;display number
003C CD INT 21H
.EXIT ;exit to DOS
END ;end of file
Example 6.4

23. Example 6.5 .MODEL SMALL ;select small model
.MODEL SMALL ;select small model
DATA ;start of data segment
R TABLE DW ONE ;define lookup table
R DW TWO
R DW THREE
CODE ;start code segment
.STARTUP ;start of program
0017 TOP:
0017 B MOV AH,1 ;read key into AL
0019 CD INT 21H
001B 2C SUB AL,31H ;convert to biner
001D 72 F8 JB TOP ;if below ‘1’ typed
001F 3C CMP AL,2
F4 JA TOP ;if above ‘3’ typed
0023 B MOV AH,0 ;calculate table address
C0 ADD AX,AX
F0 ADD SI,AX
0029 FF A R JMP TABLE [SI] ;jump to ONE, TWO or THREE
002D ONE:
002D B MOV DL,’1’ ;load DL with ‘1’
002F EB JMP BOT
0031 TWO:
0031 B MOV DL,’2’ ;load DL with ‘2’
0033 EB JMP BOT
0035 THREE:
0035 B MOV DL,’3’ ;load DL with‘3’
0037 BOT:
0037 B MOV AH,2 ;display ONE, Two or THREE
0039 CD INT 21H
.EXIT ;exit to DOS
END ;end of file

24. Example 6.6 ;through the SI register ; 0017 SCAN PROC NEAR
;A procedure that searches a table of 100 bytes for 0AH
;The address, TABLE, is transferred to the procedure
;through the SI register ;
0017 SCAN PROC NEAR
0017 B MOV CX,100 ;load count of 100
001A B0 0A MOV AL,0AH ;load AL wiyh 0AH
001C FC CLD ;select increment
001D F2/AE REPNE JMP NEXT ;test 100 bytes fir 0AH
001F F9 STC ;set carry for not found
0020 F3 01 JCXZ NOT_FOUND ;if not found
0022 F8 CLC ;clear carry if found
0023 NOT_FOUND:
0023 C3 RET ;return from procedure
SCAN ENDP

25. Example 6.7 0025 L1: ;and stores the result over top of data in BLOCK2
;A program that sums the contens of BLOCK1 and BLOCK2
;and stores the result over top of data in BLOCK2
;through the SI register ;
.MODEL SMALL ;select small model
DATA ;start of data segment
[ BLOCK1 DW DUP (?) ;100 bytes for BLOCK1
0000 ]
00C [ BLOCK2 DW DUP (?) ;100 bytes for BLOCK2
CODE ;start of code segment
.STARTUP ;start of program
0017 8C D8 MOV AX,DS ;overlap DS and ES
0019 8E C0 MOV ES,AX
001B FC CLD ;select increment
001C B MOV CX,100 ;load count 100
001F BE 0000 R MOV SI,OFFSET BLOCK1 ;address BLOCK1
0022 BF 0000 R MOV DI,OFFSET BLOCK2 ;address BLOCK2
L1:
0025 AD LODSW ;load AX with BLOCK1
: ADD AX,ES:[DI] ;add BLOCK2 data to AX
0029 AB STOSW ;store sum in BLOCK2
002A E2 F9 LOOP L1 ;repeat 100 times
.EXIT ;exit to DOS
END ;end file

26. Example 6.8(a) Example 6.8(b) MOV AH,30H INT 21 H
; Inti Program Sequence
MOV AH,30H
INT 21 H
.IF AL<3 && AH<30
MOV AH,4CH
INT 21H
.ENDIF
Example 6.8(b)
; Diagram file bahasa Mesin pada contoh 6.8 (a) ;
0000 B MOV AH,30H
0002 CD 21 INT 21H
.IF AL<3 && AH<30
0004 3C 03 * CMP AL,003H
* JAE @c0001
FC 1E* CMP AH,01EH
000B *
000D B4 4C MOV AH,4CH
000F CD 21 INT 21H
.ENDIF
0011

27. Example 6.9
 [c2]

28. Example 6.10
; Program yang membaca sebuah key dan menyimpan dalam Hexadecimal
; Nilai pada memori lokasdi TEMP.
.Mode Small : Pilih model SMALL
Data : Memulai data segmen
TEMP DB? : Define TEMP
CODE : Awal Code segment
STARTUP : Start program
0017 B MOV AH, : Pembacaan key
0019 CD INT 21H
. IF AL>=’a’ && AL<=’f’ : Bila dibagian sisi bawah
C SUB AL,57H
.ELSEIF AL>=’A’ && AL<=’F’ : bila bagian sisi atas
002F 2C ELSE : Hal lain
C SUB AL,30H
.ENDIF
A R MOV TEMP, AL
.EXIT : Keluar DOS
END : Akhir file