Code Conversion

CSC-167 · Semester II · Microprocessor

intermediate30 minAvailable

Code Conversion

HEX ↔ packed-BCD conversion, step by step.

Decimal value (0-255)173

Result

0xAD ↔ 0173 BCD

Conversion steps

1HEX value: 173 decimal = 0xAD
2Split into decimal digits: 1 (hundreds), 7 (tens), 3 (units)
3Pack each digit as a nibble: 1 -> 0001, 7 -> 0111, 3 -> 0011
4BCD result: 0173 (store: DAA adjusts after ADD for packed BCD)

Lab brief

Objective

Write 8085 programs that convert between BCD, hexadecimal and ASCII representations and check the results by hand and by simulation.

Theory & concepts

Microprocessors exchange data in several codes. Binary (hex) is what the ALU computes with; BCD stores each decimal digit in a nibble; ASCII uses 7-bit codes for characters. Display drivers and keyboards need conversions, so every 8085 kit has standard conversion routines.

Rendering diagram…

BCD-to-hex and hex-to-ASCII conversions

Value	Hex	BCD	ASCII (hex code)
5	05H	05H	35H
12	0CH	12H	31H 32H
255	FFH	255H (3 nibbles)	32H 35H 35H

; BCD to hex: 2-digit BCD in A -> binary value in A.
        MVI B, A       ; copy BCD to B
        ANI 0F0H       ; keep the tens digit only
        RRC            ; shift right to move tens into low nibble
        RRC
        RRC
        RRC            ; now tens digit (0-9) in low nibble
        MOV C, A       ; C = tens
        MOV A, B       ; original BCD
        ANI 0FH        ; keep the units digit
        ADD A          ; units x 2
        MOV B, A       ; save units x 2
        ADD A          ; units x 4
        ADD A          ; units x 8
        ADD B          ; units x 10
        ADD C          ; + tens = full binary value
        HLT

; Simpler textbook form:
;   BCD value = (tens x 10) + units
; Compute tens x 10 as (tens x 8) + (tens x 2).

; Hex digit (0-F) in low nibble of A -> ASCII in A.
; 0-9 -> 30H-39H, A-F -> 41H-46H
        ANI 0FH        ; mask to one digit
        CPI 0AH        ; compare with 10
        JC  NUM        ; digit 0-9: add 30H
        ADI 37H        ; digit A-F: ASCII 'A' is 41H = 0AH + 37H
        JMP DONE
NUM:    ADI 30H        ; ASCII '0' is 30H
DONE:   HLT

Input 0AH -> output 41H ('A'); input 07H -> output 37H ('7').

Prerequisites

Lesson: Logic, branch and machine-control instructions
Lesson: Data transfer and arithmetic instructions

Instructions

1Enter the BCD-to-hex program and run it with A = 37H (37 decimal).
2Verify the result is 25H (37 decimal) in the accumulator.
3Enter the hex-to-ASCII program and test A = 0AH, 05H and 0FH.
4Confirm the outputs 41H, 35H and 46H in the accumulator.
5Trace each program in single-step mode and note how shifts multiply by ten.

Tasks

Convert three BCD values
Run BCD-to-hex with 12H, 37H and 99H; confirm 0CH, 25H and 63H.
Convert three hex digits
Run hex-to-ASCII with 03H, 0AH and 0FH.
Explain the shift trick
Explain how RRC and ADD A implement multiplication by ten.

Self-check

All BCD-to-hex outputs are correct for the test values.
All hex-to-ASCII outputs are correct, including digits above 9.
The student can explain the multiply-by-ten construction.

Environment

8085 assembleraccumulator and flag viewsingle-step executionmemory editor

Code Conversion · Microprocessor · Learn CSIT