Lab 5 Part C

• Write to the screen a character string that uses a ‘$’ to indicate the end of the string. Do not write the ‘$’ to the screen. Use DOS Interrupt 21h Function 2 to write each character of the string to the screen.

How to Compare

• Want to compare each character in the string to ‘$’ character. – If not equal, write character– Else, exit program

• Alternatively,– If equal, exit program– Else, write character

Moving Through a String of Characters

Use a pointer register to point to the beginning of the string. (SI)

Loop through the string one character at a time. (INC SI)

Writing a String to the Screen

Move value pointed to into register DL

Is Equal to ‘$’’

Write character on screen

NO

Exit Program YES

S

Increment pointer register SI

Move beginning of string into pointer register SI

Instructions for Writing to ScreenExiting Program

Writing to the screen using Interrupt 21h

(the character to be written must be stored in DL)

Mov ah, 2 ;put function number in ah

Int 21h ;call interrupt 21h

Exiting program

Mov ah, 4Ch ;put function number in ah

Int 21h ;call interrupt 21h

Compare Loop

MOV SI, OFFSET string1

LP1: MOV DL, [SI] ;Note DL

CMP DL, ‘$’

JE Exit

……. ;print here

INC SI

JMP LP1

Put code inside template.codeMain proc

mov ax, @data ;initialize data segmentmov ds, axmov SI, offset string1 ;SI is pointer to string1

LP1: mov dl, [SI] ;save char in dlcmp dl, ‘$’ ;compare char to ‘$’je exitinc SI ;point to next charmov ah, 2 ;print charint 21hjmp LP1 ;end of loop

EXIT: mov ah, 4Ch ;exit programint 21h

Main endpEND Main

Writing a Character String in UpperCase, LowerCase or Reversing Case

• OR each Character with 20h creates uppercase characters

• AND each character with DFh creates Lowercase characters

• XOR each character with 20h reverses the case. Uppercase becomes lowercase; Lowercase becomes uppercase

Lab 5 Part D

• Write to the screen an 8-bit number that is stored in memory. Write the number in binary. Write each binary bit as a character using DOS Interrupt 21h, function 2.

Dealing with Bits

• To look at each bit, we need to use instructions that deal with bits. The shift and rotate instructions allow us to evaluate individual bits.

• shr bl, 1

the lsb is placed in the carry flag

• shl bl, 1

The msb is placed in the carry flag

Jumps Based on Status

• A conditional jump based on the value of the carry status flag would then be taken

What is a 1 and 0 in ASCII

0 = 30h

1 = 31h

Therefore, move dl, 30h if carry flag = 0

Move dl, 31h if carry flag = 1

Pseudocode

Move number into register from memorySetup loop = 8 to access each bit individuallyLP1: Shift each bit into carry flag

Jump to Is_One procedure if carry = 1Else move into DL, 30hJump to print label

Is_One: Move into DL, 31hPrint: Use DOS interrupt Function 2 to write char

to screen.Loop back to LP1 if CX >= 0

Exit

Writing a Number in Binary

Shift BL by one to get msb into carry flag

Carry Equals 1?

Move 30h into DL

NO

Mov 31h into DLYES

S

Write character on screen

Move 8 into CX (dealing with a byte value)

Move Number into BL

Lab 6 Part A

• Write a number in hexadecimal. Use DOS interrupt 21h, function 2 to write a character to the screen.

• Write a CRLF procedure since this function is used so often. Use the DOS interrupt 21h, function 2 to write the appropriate characters to generate a CR and LF on the screen.

Converting a Decimal Digit to a Hex Digit

• You know how to change a bit that represents a decimal value 0-1 to an ASCII character value. Add 30h to the value.

• Use 4 bits to represent the decimal values 0-9.

• Compare the value to see if the value is greater than 9. If so, it must be a character A-F.

How do You only look at 4 bits out of a Byte Size Register?

• MASK the upper four bits of the byte. Be sure to save the value of the byte before you mask it; the register will get changed.

• Use AND 0Fh to keep the lower four bits; hide the upper four bits of the 8 bit register value.

• You now have a value between 0 and 15.• Compare to 9

– Equal or less – no change (add 30h to get ASCII value)

– Greater => must be a character. (add 37h to get ASCII value)

Convert_to_Hex Procedure (Changes the value of AL)

Convert_to_Hex procAnd al, 0FhCmp al, 9

Ja ischarAdd al, 30hjmp exit

Ischar: Add al, 37hExit: retConvert_to_Hex proc

How do you write a Carriage Return to the screen

• There is an ASCII character labelled “Carriage Return” (0Dh)

• There is an ASCII character labelled “Line Feed” (0Ah)

PCRLF Procedure

Pcrlf Proc

mov ah, 2

mov dl, 0Ah

int 21h

mov dl, 0Dh

int 21h

ret

Print_char Procedure

Print_char proc

mov ah, 2

int 21h

ret

Print_char endp

Main ProgramMain proc

mov ax, @datamov ds, axpushaxcall convert_to_hexmov dl, alcall print_charcall pcrlfmov ax, 4c00hint 21h

Main endp

Order of Procedures

Main proc…..

Main endpConvert_to_Hex proc

…..Convert_to_Hex endpPrint_char proc

…..Print_char endpPcrlf proc

…..Pcrlf endpEnd Main

ASCII Decimal String to Hex Conversion

• Given a two digit unsigned decimal string, with each digit in ASCII, convert it to the correct 8-bit value.

Conversion from ASCII

• The first ASCII value is the 10’s digit. Subtract 30h from the value to get the unsigned binary (or hex) representation, then multiply it by 10.

• Subtract 30h from the second digit, and then add it to the result of the first operation.