Terms used for addressing modes. [intel 8085]

Question

In the documentation of a processor I am working on it says :

Operand addressing modes available are implied, register, immediate, direct and register indirect (using the BC, DE and HL register pairs as 16-bit pointers to memory).

Can someone please explain the difference between these addressing modes in laymans terms, or perhaps post up a link where I may learn?

Thanks a lot.

Answer 1

• implied: the instruction uses no register, or the register mapping is fixed, so no registers are named in the assembler mnemonic
• register: the instruction operates on the contents of a register
• immediate: the data is part of the instruction stream, this can be used for constants only
• direct: the address of the data is part the instruction stream (i.e. data at a fixed address)
• register indirect: the address of the data is found in a register, or a set of registers taken together.

Answer 2

Here is rather full assembler manual of 8080/8085:

[PDF] Intel 8080/8085 Assembly Language Programming
File Format: PDF/Adobe Acrobat

ISIS-II 8080/8085 IAACRO ASSEMBLER OPERATOR'S MANUAL, 9800292. When you activate the
assembler, you have the option of specifying a number of controls. ...

http://krchowdhary.com/intel-8085.pdf (5.5 MB)

You need section 1-15 (pdf page 27)

Answer 3

Implied means that the opcode does not require the programmer to explicitely state the source and address; they are implied in the statement. For instance LDA always loads the accumulator, the accumulator is the implied destination address for all LDA opcodes.

Register means that the source or destination is a processor register rather than a memory location. for instance MOV r1,r2 is a register-to-register transfer which does not involve a memory fetch beyond the opcode.

Immediate means a value that is in memory in the code store, typically a value immediately following the opcode. I.E.: MOV r,n. The memory address reference for the value load is the program counter.

Direct means that a memory address is provided after the opcode and the processor first loads that memory address into the address register then fetches from that memory location. The memoryy address reference is supplied in following the opcode, and execution first requires loading the two byte address then loading the memory value from that location.

Indirect means that the address is stored in one of the registers that can be used for that purpose. As it is just a register-to-register transfer to load the address registers for the memory fetch, it is faster than using direct addressing.