How to encode ASCII text in binary opcode instructions?

Question

I do not need a refresher on anything. I am generally asking how would one encode a data string within the data segment of a binary file for execution on bare metal.

Purpose? Say I am writing a bootloader, and I need to define the static data used in the file to represent the string to move to a memory address, for example, with the Genesis TMSS issue.

I assume that binary encoding of an address is literally-translated-as binary equivalent to its hexadecimal representation in Motorola 68000 memory-mapping, so that's not an issue for now.

The issue is ... how do I encode strings/characters/glyphs in binary code to encode within an M68000k opcode? I read the manual, references, etc., and none quite touch this(from what I read through).

Say I want to encode move.l #'SEGA', $A14000. I would get this resulting opcode (without considering how to encode the ASCII characters):

0010 1101 0100 0010 1000 0000 0000 0000

Nibble 1 = MOVE LONG, Nibble 2 = MEMORY ADDRESSING MODE, Following three bytes equals address.

My question is, do I possibly encode each string in literal ASCII per character as part of the preceeding MAM nibble of the instruction?

I am confused at this point, and was hoping somebody might know how to encode data text within an instruction.

Answer 1

Well I had experienced programming in 4 different assembly languages,and Motorola M68HC11 is one of them.In my experience, ASCII is just for display purposes. The CPU at low level treats everything as binary values, it cannot distinguish between ASCII characters and other characters. Although higher assembly languages like x86 support instructions like AAA(ASCII adjust for addition) which makes sure that after addition of two ASCII numbers the result is still a legal ASCII number. So mainly it's assembler dependent, if the assembler supports the instruction move.l #'SEGA', $A14000 this might work, but since you are not using an assembler and directly writing op-codes, you have to encode the ascii into binary, example ascii number '1'(0x31) will be encoded as 0000 0000 0011 0001 in 16 bit representation. Also in my experience there's no machine code which can move the whole string. SO in microcode the first character is fetched, then copied to destination address, then second character is fetched and copied it into second location and so on.. Assuming that instruction size is 32 bits long and immediate addressing mode is supported the first two nibbles would suggest the move instruction and the immediate addressing type, next two nibbles would be the binary encoded character and remaining would be the address you want to copy it to. Hope this helps