Added sketch for an interrupt driven keyboard routine

src/example/char_int.asm

@@ -0,0 +1,57 @@
+;;Query the keyboard in an interrupt routine.
+        ORG     38H         ;Assemble into the interrupt routine
+        DI                  ;Disable interrupts
+        EX      AF,AF'      ;Save the state of the AF register
+        EXX                 ;Save the state of any other registers
+        CALL    INTR        ;Call the interrupt handler
+        EXX                 ;Restore the state of each register
+        EX      AF,AF'      ;Restore the accumulator
+        EI                  ;Enable the interrupts
+        RET                 ;Return to caller
+        ORG     100H        ;Tell the assembler to start at addr 0x100
+START:  IM 1                ;Set interrupt mode 1
+        LD      HL,GREET    ;Load the start address of the banner
+        LD      B,15H       ;Set the length of the banner text
+        LD      DE,0H       ;Start output in the top left corner
+        CALL    84CDH       ;Call str_out
+;; TODO: Add code
+        RET                 ;End the program
+INTR:   IN      A,(16H)     ;Read the interrupt register
+        AND     1H          ;Check if this is a keyboard interrupt
+        RET     Z           ;Return if the bit was set
+        CALL    8881H       ;Check the keycode of the pressed key
+        CP      51H         ;Checks if the pressed key was "ON"
+        JR      NZ,CNT      ;If the key was not "ON" continue
+        LD      HL,EXIT     ;Load the exit marker
+        LD      A,1H        ;Set the exit value to 1
+        LD      (HL),A      ;Set the exit flag
+        RET                 ;Return
+CNT:    LD      HL,BUF      ;Load the buffer address
+        CALL    BTOHEX      ;Convert the byte in A into a HEX-string and store it to BUF
+        LD      B,2         ;Set the length for BUF
+        LD      DE,15H      ;Position cursor at row 0, col 22
+        CALL    84CDH       ;Print the keycode
+        RET                 ;Return from INT
+;;Convert the contents of A into a hex-string and stores the result in the buffer pointed in HL
+BTOHEX: PUSH    AF          ;Store A onto the stack for later use
+        LD      B,4         ;Init a loop for shifting the register
+LP1:    SRL     A           ;Shift the MSN down to the least significant Nibble
+        DJNZ    LP1         ;Shift again, until all 4 bits where shifted.
+        CALL    HEXDGT      ;Converts the LSN to hex
+        INC     HL          ;Point HL to the next byte in buffer
+        POP     AF          ;Retrieve the original A
+        AND     0FH         ;Cut the first digit of the hex no.
+        CALL    HEXDGT      ;Call HEXDIGIT again for the second digit.
+        LD      HL,BUF      ;Restore the buffer address
+        RET
+;;Converts the least significant nibble in A into a ASCII hex-digit and stores the result
+HEXDGT: CP      0AH         ;Check if the number is smaller than 10
+        JR      C,SKIP      ;Skip the next instruction, if it is
+        ADD     A,07H       ;If the value is >10, we need to skip the chars between '9' and 'A'
+SKIP:   ADD     A,30H       ;Converts the number into its ASCII digit
+        LD      (HL),A      ;Store the digit in the correct location.
+        RET        
+GREET:  DB      'Test keyboard input: '
+BUF:    DS      2           ;Define a 2 byte bufferto store the keycode in ASCII
+EXIT:   DB      0           ;Sets the end marker in the code      
+        END                 ;Signals the code end to the assembler