Wednesday, January 7, 2009

The 8085 Instruction Set

The 8085 Instruction Set

Instructions can be categorized according to their method of addressing the hardware registers and/or memory.

Implied Addressing:
The addressing mode of certain instructions is implied by the instruction’s function. For example, the STC (set carry flag) instruction deals only with the carry flag, the DAA (decimal adjust accumulator) instruction deals with the accumulator.

Register Addressing:
Quite a large set of instructions call for register addressing. With these instructions, you must specify one of the registers A through E, H or L as well as the operation code. With these instructions, the accumulator is implied as a second operand. For example, the instruction CMP E may be interpreted as 'compare the contents of the E register with the contents of the accumulator.

Most of the instructions that use register addressing deal with
8-bit values. However, a few of these instructions deal with 16-bit register pairs. For example, the PCHL instruction exchanges the contents of the program counter with the contents of the H and L registers.

Immediate Addressing:
Instructions that use immediate addressing have data assembled as a part of the instruction itself. For example, the instruction CPI 'C' may be interpreted as ‘compare the contents of the accumulator with the letter C. When assembled, this instruction has the hexadecimal value FE43. Hexadecimal 43 is the internal representation for the letter C. When this instruction is executed, the processor fetches the first instruction byte and determines that it must fetch one more byte. The processor fetches the next byte into one of its internal registers and then performs the compare operation.

Notice that the names of the immediate instructions indicate that they use immediate data. Thus, the name of an add instruction is ADD; the name of an add immediate instruction is ADI.

All but two of the immediate instructions uses the accumulator as an implied operand, as in the CPI instruction shown previously. The MVI (move immediate) instruction can move its immediate data to any of the working registers including the accumulator or to memory. Thus, the instruction MVI D, OFFH moves the hexadecimal
value FF to the D register.

The LXI instruction (load register pair immediate) is even more unusual in that its immediate data is a 16-bit value. This instruction is commonly used to load addresses into a register pair. As mentioned previously, your program must initialize the stack pointer; LXI is the instruction most commonly used for this purpose. For example, the instruction LXI SP,3OFFH loads the stack pointer with the hexadecimal value 30FF.


Direct Addressing:
Jump instructions include a 16-bit address as part of the instruction. For example, the instruction JMP 1000H causes a jump to the hexadecimal address 1000 by replacing the current contents of the program counter with the new value 1000H.

Instructions that include a direct address require three bytes of storage: one for the instruction code, and two for the 16-bit address

Register Indirect Addressing:
Register indirect instructions reference memory via a register pair. Thus, the instruction MOV M,C moves the contents of the C register into the memory address stored in the H and L register pair. The instruction LDAX B loads the accumulator with the byte of data specified by the address in the B and C register pair.

Combined Addressing Modes:
Some instructions use a combination of addressing modes. A CALL instruction, for example, combines direct addressing and register indirect addressing. The direct address in a CALL instruction specifies the address of the desired subroutine; the register indirect address is the stack pointer. The CALL instruction pushes the current contents of the program counter into the memory location specified by the stack pointer.

Timing Effects of Addressing Modes:
Addressing modes affect both the amount of time required for executing an instruction and the amount of memory required for its storage. For example, instructions that use implied or register addressing, execute very quickly since they deal directly with the processor’s hardware or with data already present in hardware registers. Most important, however is that the entire instruction can be fetched with a single memory access. The number of memory accesses required is the single greatest factor in determining execution timing. More memory accesses therefore require more execution time. A CALL instruction for example, requires five memory accesses: three to access the entire instruction and two more to push the contents of the program counter onto the stack.

The processor can access memory once during each processor cycle. Each cycle comprises a variable number of states. (See below and the appendix of “USING THE SDK-85 MICROPROCESSOR TRAINER”). The length of a state depends on the clock frequency specified for your system, and may range from 480 nanoseconds to 2 microseconds. Thus, the timing for a four state instruction may range from 1.920 microseconds through 8 microseconds. (The 8085 have a maximum clock frequency of 5 MHz and therefore a minimum state length of 200 nanoseconds.)

Instruction Naming Conventions:
The mnemonics assigned to the instructions are designed to indicate the function of the instruction. The instruc­tions fall into the following functional categories:

Data Transfer Croup:
The data transfer instructions move data between registers or between memory and registers.

MOV Move
MVI Move Immediate
LDA Load Accumulator Directly from Memory
STA Store Accumulator Directly in Memory
LHLD Load H & L Registers Directly from Memory
SHLD Store H & L Registers Directly in Memory

An 'X' in the name of a data transfer instruction implies that it deals with a register pair (16-bits);

LXI Load Register Pair with Immediate data
LDAX Load Accumulator from Address in Register Pair
STAX Store Accumulator in Address in Register Pair
XCHG Exchange H & L with D & E
XTHL Exchange Top of Stack with H & L

Arithmetic Group:
The arithmetic instructions add, subtract, increment, or decrement data in registers or memory.

ADD Add to Accumulator
ADI Add Immediate Data to Accumulator
ADC Add to Accumulator Using Carry Flag
ACI Add Immediate data to Accumulator Using Carry
SUB Subtract from Accumulator
SUI Subtract Immediate Data from Accumulator
SBB Subtract from Accumulator Using Borrow (Carry) Flag
SBI Subtract Immediate from Accumulator Using Borrow (Carry) Flag
INR Increment Specified Byte by One
DCR Decrement Specified Byte by One
INX Increment Register Pair by One
DCX Decrement Register Pair by One
DAD Double Register Add; Add Content of Register
Pair to H & L Register Pair

Logical Group:
This group performs logical (Boolean) operations on data in registers and memory and on condition flags.

The logical AND, OR, and Exclusive OR instructions enable you to set specific bits in the accumulator ON or OFF.

ANA Logical AND with Accumulator
ANI Logical AND with Accumulator Using Immediate Data
ORA Logical OR with Accumulator
OR Logical OR with Accumulator Using Immediate Data
XRA Exclusive Logical OR with Accumulator
XRI Exclusive OR Using Immediate Data

The Compare instructions compare the content of an 8-bit value with the contents of the accumulator;

CMP Compare
CPI Compare Using Immediate Data

The rotate instructions shift the contents of the accumulator one bit position to the left or right:

RLC Rotate Accumulator Left
RRC Rotate Accumulator Right
RAL Rotate Left Through Carry
RAR Rotate Right Through Carry

Complement and carry flag instructions:

CMA Complement Accumulator
CMC Complement Carry Flag
STC Set Carry Flag

Branch Group:
The branching instructions alter normal sequential program flow, either unconditionally or conditionally. The unconditional branching instructions are as follows:

JMP Jump
CALL Call
RET Return

Conditional branching instructions examine the status of one of four condition flags to determine whether the specified branch is to be executed. The conditions that may be specified are as follows:

NZ Not Zero (Z = 0)
Z Zero (Z = 1)
NC No Carry (C = 0)
C Carry (C = 1)
PO Parity Odd (P = 0)
PE Parity Even (P = 1)
P Plus (S = 0)
M Minus (S = 1)

Thus, the conditional branching instructions are specified as follows:

Jumps Calls Returns
C CC RC (Carry)
INC CNC RNC (No Carry)
JZ CZ RZ (Zero)
JNZ CNZ RNZ (Not Zero)
JP CP RP (Plus)
JM CM RM (Minus)
JPE CPE RPE (Parity Even)
JP0 CPO RPO (Parity Odd)

Two other instructions can affect a branch by replacing the contents or the program counter:

PCHL Move H & L to Program Counter
RST Special Restart Instruction Used
with Interrupts

Stack I/O, and Machine Control Instructions:
The following instructions affect the Stack and/or Stack Pointer:

PUSH Push Two bytes of Data onto the Stack
POP Pop Two Bytes of Data off the Stack
XTHL Exchange Top of Stack with H & L
SPHL Move content of H & L to Stack Pointer

The I/0 instructions are as follows:

IN Initiate Input Operation
OUT Initiate Output Operation

The Machine Control instructions are as follows:
EI Enable Interrupt System
DI Disable Interrupt System
HLT Halt
NOP No Operation

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