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ASM 8085 Cheat Sheet

Registers

A (Accum­­ul­ator)
After performing arithm­etical or logical operat­ions, the result is stored here
BC
Genera­l-p­urpose register that is capable of storing 16-bit data (B - 8-bit) (C - 8-bit)
DE
Genera­l-p­urpose register that is capable of storing 16-bit data (D - 8-bit) (E - 8-bit)
HL
Usually used to store a memory address Ex. (00 - H) (36 - L). It also creates a hypoth­etical register labeled as 'M'
Note: The general purpose registers in 8085 processors are B, C, D, E, H and L.
Each register can hold 8-bit data.
They can work in pairs such as B-C, D-E and H-L to store 16-bit data.
The H-L pair works as a memory pointer.
  

Flag Registers

S (Sign Flag)
If MSB bit = 0 then the number is positive, else it is negative.
Z (Zero Flag)
If an operation performed in A results 0 value of entire 8-bits then zero flag is set, else it resets.
AC (Auxiliary Carry Flag)
If an operation performed in A generates the carry from lower nibble (D0 to D3) to upper nibble (D4 to D7) AC flag is set, else it resets.
P (Parity Flag)
If the result contains even no. of ones this flag is set and for odd no. of ones this flag is reset.
CY (Carry Flag)
If an operation performed in A generates the carry from D7 to next stage then CY flag is set, else it is reset.
Note – The Auxiliary Carry flag register in 8085 is the only flag not accessible by the user.
  

Memory Registers

PC (Program Counter)
Stores the address of the next instru­ction to be executed.
SP (Stack Pointer)
Stack pointer maintains the address of the last byte that is entered into stack.
PSW (Program Status Word)
It combines the Accumu­lator register with all the flag registers in a 16-bit format
Note: A stack is nothing but a portion of RAM (Random access memory).
Each time when the data is loaded into stack, Stack pointer gets decrem­ented.
Conversely it is increm­ented when data is retrieved from stack.
A stack is treated as a 16-bit entry and it consumes 2 locations from a memory for 1 entry.
A stack requires a 16-bit register to be pointed to.

Machine Cycles

Opcode Fetch Machine Cycle
4T or 6T
Memory Read Machine Cycle
3T
Memory Write Machine Cycle
3T
I/O Read Machine Cycle
3T
I/O Write Machine Cycle
3T
Most of the time, it's just 4T for the Opcode Fetch, there are only a few commands that require 6T
  

Addressing Modes

Direct Addressing
In this addressing mode, the address of the operand (data) is given in the instru­ction itself.
Register Addressing
In register addressing mode, the operand is in one of the general purpose registers. The opcode specifies the address of the regist­er(s) in addition to the operation to be performed.
Register Indirect Addressing
In Register Indirect mode of addres­sing, the address of the operand is specified by a register pair.
Immediate Addressing
In this addressing mode, the operand is specified within the instru­ction itself.
Implicit Addressing
There are certain instru­ctions which operate on the content of the accumu­lator. Such instru­ctions do not require the address of the operand.
Examples:
1. Direct Addres­sing:
STA 2400H
2. Register Addres­sing:
MOV A, B
3. Register Indirect Addressing
LXI H, 2500 H
MOV A, M
4. Immediate Addressing
LXI H, 2500
5. Implicit Addressing
CMA, RAL, RAR, etc.
           
 

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  • Published: 6th May, 2020
  • Last Updated: 13th May, 2020

Comments

A cheat sheet about 8085 ASM, lovely !

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