Computer Science AS · Processor fundamentals. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Machine code is the binary code a processor runs directly. Assembly language writes the same instructions with short names (mnemonics) such as LDD and ADD, and with labels in place of addresses. Each assembly instruction has an opcode, which says what to do, and usually an operand. One assembly instruction normally becomes one machine code instruction.
An assembler does the translation. A two-pass assembler reads the program twice, so that a label can be used before the line where it is defined.
The addressing mode tells the processor how to use the operand: as the value itself, as an address, or as a way of working out an address.
Students load the address in place of its contents. LDM #60 puts the number 60 in ACC, but LDD 60 puts the contents of address 60 in ACC.
Answer: Address 62 contains 7.
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