Designing Embedded Hardware, 2nd Edition

Machine Code

Everything that a processor deals with is expressed as numbers in memory. That applies to data, and to software as well. Your compiled C program is converted to a sequence of numbers that is meaningful to the processor as a sequence of instructions. A program that a processor (in this case, a 68HC11) can execute might look something like:

   86 41 B7 01 00 BD 02 00

This is known as machine code, and each numeric instruction is called an opcode. Humans find such programs very hard to write and even harder to understand. To make this easier to cope with, we use a notation called assembly language. Assemblylanguage instructions equate directly to their machine code counterparts. Since machine code is difficult to read and write (for a human), a mnemonic is used to represent the opcode. For example, the 68HC11 instruction 86 41 is more easily understood by its mnemonic LDAA #$41. (The dollar sign [$] represents a hex value in some assembly languages.) This is still a bit cryptic, so we usually add comments on the righthand side to help us follow what is going on. The previous machine code sequence written as 68HC11 assembly would be:

Machine code   Assembly    Comment
86 41          LDAA #$41   ; anything after a semi-colon
B7 01 00       STAA $0100  ; is a comment
BD 02 00       JSR $0200   ;

LDAA #$41 means "load accumulator A with the immediate value (signified by the #) of 0x41." Immediate means that the operand is to be treated as a number, rather than as a pointer to an address. With some processors, ...