8086 ALP: alternate case display

Method: read the word with DOS function 0Ah, clear the screen with INT 10h (function 06h), then scan each character. Letters are forced to lower case for the 1st, 3rd, ... letter and to upper case for the 2nd, 4th, ... letter; BL toggles between 0 (next is lower) and 1 (next is upper). Non-letters are displayed unchanged and do not change the toggle.

.MODEL SMALL
.STACK 100H

DISPSTR MACRO MSG            ; display a '$'-terminated string
        LEA  DX,MSG
        MOV  AH,09H
        INT  21H
        ENDM

CLRSCR  MACRO                ; clear screen and home the cursor
        MOV  AX,0600H        ; scroll whole window (AL=0 -> blank it)
        MOV  BH,07H          ; normal attribute
        MOV  CX,0000H        ; upper-left corner (row 0, col 0)
        MOV  DX,184FH        ; lower-right corner (row 24, col 79)
        INT  10H
        MOV  AH,02H          ; set cursor position
        MOV  BH,00H          ; page 0
        MOV  DX,0000H        ; row 0, col 0
        INT  10H
        ENDM

.DATA
    MSG1  DB 'Enter a word: $'
    BUF   DB 80, ?, 80 DUP(0)   ; DOS buffered-input area (max, actual, chars)
    CRLF  DB 13,10,'$'

.CODE
MAIN PROC
    MOV  AX,@DATA
    MOV  DS,AX

    DISPSTR MSG1
    LEA  DX,BUF            ; read string (function 0Ah)
    MOV  AH,0AH
    INT  21H
    CLRSCR                 ; clear window

    LEA  SI,BUF+2          ; SI -> first character
    MOV  CL,BUF+1          ; CL = number of characters typed
    MOV  CH,0
    JCXZ EXIT              ; nothing typed
    MOV  BL,0              ; BL=0: next letter must be lower case
NEXT:
    MOV  DL,[SI]           ; get character
    CMP  DL,'A'
    JB   SHOW              ; below 'A' : not a letter
    CMP  DL,'Z'
    JBE  LETTER            ; 'A'..'Z'
    CMP  DL,'a'
    JB   SHOW              ; between 'Z' and 'a'
    CMP  DL,'z'
    JA   SHOW              ; above 'z'
LETTER:
    OR   DL,20H            ; make it lower case
    CMP  BL,0
    JE   FLIP              ; lower case wanted
    AND  DL,0DFH           ; upper case wanted
FLIP:
    XOR  BL,1              ; toggle for next letter
SHOW:
    MOV  AH,02H            ; display character in DL
    INT  21H
    INC  SI
    LOOP NEXT
EXIT:
    MOV  AH,4CH            ; return to DOS
    INT  21H
MAIN ENDP

END MAIN

Example: input Microprocessor → output mIcRoPrOcEsSoR.