The 8051

 
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  A Basic Instruction Set for a Basic Microprocessor
   
  Before we start to look at the 8051 instruction set, it would be better to take a fictitious, basic microprocessor and examine a limited number of instructions that this microprocessor might perform.
   
 
   
  Inside the ALU there is a special register called the accumulator. In our assembly language, we will refer to this 8-bit register as A.
   
  To facilitate this we will also invent a basic assembly language which we will use to demonstrate the instruction set.
   
 
Assembly HEX Equivalent Operation Size of Instruction
INC A E4 (A) <- (A) + 1 1 byte
DEC A E5 (A) <- (A) - 1 1 byte
LD A, data 3E, data (A) <- data 2 bytes
JP, address D6, address (PC) <- address 3 bytes
   
  The instruction set consists of four instructions. The first one increases the contents of A by one. The second decreases the contents of A by one. The third loads the data into A and the fourth jumps to address by storing address in the PC.
   
  We could write a simple program that uses all four instructions.
   
  LD A, 05H
  INC A
  INC A
  DEC A
  JP, 0000H
   
  Assembling the program above involves taking the assembly language and changing it into HEX code.
   
 
  0 1 2 3 4 5 6 7
0000 3E 05 E4 E4 E5 D6 00 00
   
  The machine code (ie, the HEX equivalent of our program) is stored in ROM, starting at address 0000H. Therefore, the program loads 05H into A, increases A by one twice and then decreases A by one once. It then jumps back to 0000H (ie, the start of the program). This program would loop forever.
   
  Try out EdSim51's Execution Cycle Simulation - part 2.
   
   
  Conditional Jump
  One more instruction is now added to our basic instruction set - a conditional jump, as detailed in the table below.
   
 
Assembly HEX Equivalent Operation Size of Instruction
INC A E4 (A) <- (A) + 1 1 byte
DEC A E5 (A) <- (A) - 1 1 byte
LD A, data 3E, data (A) <- data 2 bytes
JP, address D6, address (PC) <- address 3 bytes
JP Z, address D7, address

if (A) = 0 (PC) <- address
if (A) != 0 (PC) <- (PC) + 3

 3 bytes
   
 
JP Z, address can is described as follows: if the contents of A are zero, jump to address, otherwise continue with next instruction.
   
 
Now we write a second program.
   
 
ORG 0130H
  LD A, 03H
  DEC A
  JP Z, 0130H
  JP 0132H
   
  The machine code equivalent of the above program is:
   
 
  0 1 2 3 4 5 6 7 8
0130 3E 03 E5 D7 30 01 D6 32 01
   
  Note the program starts at location 0130H. This was specified by the ORG statement. We stated earlier that the program counter is loaded with 0000H on power up. However, the system can be designed so that an address other than 0000H is placed in the PC on power up. This is illustrated in the second example below.
   
  Try out EdSim51's Execution Cycle Simulation - part 2 (with conditional jump).
   
   
 
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