Now the program is ready to be run inside the debugger. The debugger shows the values of registers, flags, stack, our code, and one or two areas of the system memory as data. Debugger allows us to step our program one instruction at a time and observe its effect on the registers and program data. The details of using the AFD debugger can be seen from the AFD manual.

After loading the program in the debugger observe that the first instruction is now at 0100 instead of absolute zero. This is the effect of the org directive at the start of our program. The first instruction of a COM file must be at offset 0100 (decimal 255) as a requirement. Also observe that the debugger is showing your program even though it was provided only the COM file and neither of the listing file or the program source. This is because the translation from mnemonic to opcode is reversible and the debugger mapped back from the opcode to the instruction mnemonic. This will become apparent for instructions that have two mnemonics as the debugger might not show the one that was written in the source file.

As a result of program execution either registers or memory will change. Since our program yet doesn’t touch memory the only changes will be in the registers. Keenly observe the registers AX, BX, and IP change after every instruction. IP will change after every instruction to point to the next instruction while AX will accumulate the result of our addition.

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