The other day a friend of mine asked me to code a decimal-to-hex-converter in MASM using DOS ISRs (Interrupt Service Routine) for I/O. Well, I just gave her the equivalent C code since I am not into proprietary software. But, anyway I googled for some materials related to MASM syntax and DOS ISRs for I/O [0]. Upon reading some examples of using DOS ISR 21h, I stumbled upon the use of LEA dx, var and MOV dx, OFFSET var. So, I decide to have a look into them and come up with the following conclusion.