Instruction and data format, addressing modes

Instruction format

Every 8085 instruction has an opcode (operation code) and, optionally, an operand. Depending on length, there are three formats.

In 3-byte instructions the low-order byte of the address/data is stored first, then the high-order byte.

Data format

The 8085 is an 8-bit machine; a word is 8 bits (D7 ... D0). Data can be:

• Unsigned binary: 00H to FFH (0 to 255).
• Signed binary: 2's complement; D7 is the sign bit (-128 to +127).
• BCD: packed BCD has two digits per byte (e.g. 47H = 47); DAA corrects BCD addition.
• ASCII: 7-bit code stored in a byte, e.g. 'A' = 41H.
• 16-bit data/address: stored in two memory locations, low byte at the lower address.

Addressing modes

The way the operand is specified in an instruction.

1. Immediate: the data is part of the instruction. MVI B,45H, LXI H,2050H, ADI 0FH.
2. Register (register direct): the operand is in a register. MOV A,B, ADD C.
3. Direct: the address of the operand is given in the instruction (a 16-bit memory address, or an 8-bit port address for IN/OUT). LDA 2050H, STA 3000H, IN 80H, OUT 81H.
4. Register indirect: a register pair holds the memory address of the operand. MOV A,M (address in HL), LDAX B, STAX D.
5. Implied (implicit): the operand is implied by the opcode itself, usually the accumulator. CMA, RAL, DAA, STC.