Special function registers (Bit addressable registers) 8051 RAM Memory The 8051 microcontroller consists of 256 bytes of RAM, which is divided into two ways, such as 128 bytes for general purpose and 128 bytes for special function registers (SFR) memory. CHAPTER # 4 - ADDRESSING MODES IN 8051. Upon Completion of this chapter: Sort out the 5 addressing modes of the 8051 microcontroller. Compare the addressing modes of the 8051 microcontroller. Code the 8051 Assembly Language instructions using each addressing mode. Access RAM using various addressing modes. Accessing the Special Function Registers. SETB instruction sets the bit operand to a value of 1 Can be used to carry flag or any other directly addressable bit No other flags are affected by this instruction. CPL Instruction CPL instruction is used to logically complement the value of the specified operand Example: MOV A, #55h ; A= 01010101 CPL A ; Answer will be A= 10101010 Register bank 0 is the default when the 8051 is powered up. We can switch to the other banks using PSW register. D4 and D3 bits of the PSW are used to select the desired register bank, since they can be accessed by the bit addressable instructions SETB and CLR. For example, "SETB PSW.3" will set PSW.3 = 1 and select the bank register 1. A bit addressable space in the microcontroller start from 20h to 2fh that is 16 byte field.In bit addressable space you can directly access the specific bit of that location.Out of 21 registers in 8051 11 registers are bit addressable.so instead of to worry about whole content u need to only worry about a bit.For eg. The SETB instruction sets the bit operand to a value of 1. This instruction can operate on the carry flag or any other directly addressable bit. No flags are affected by this instruction. See Also: CLR SETB bit C AC F0 RS1 RS0 OV P Bytes 2 Cycles 1 Encoding 11010010 bit Operation SETB (bit) = 1 Example SETB 63h SETB C C AC F0 RS1 RS0 OV P The 8051 micro-controller provides many functions (CPU, RAM, ROM, I/O, interrupt logic, Timer, etc) in a single package. 1. It has an 8-bit ALU and Accumulator, 8-bit Registers (one 16-bit register with special move instructions), 8-bit data bus, and 2×16-bit address bus/program counter/data pointer and related 8/11/16-bit operations. There are 5 different ways to execute this instruction and hence we say, we have got 5 addressing modes for 8051. They are 1) Immediate addressing mode 2) Direct addressing mode 3) Register direct addressing mode 4) Register indirect addressing mode 5) Indexed addressing mode. Immediate Addressing Mode Let's begin with an example. MOV A, #6AH In 8051 micro controller there are 21 Special function registers (SFR) and this includes Register A, Register B, Processor Status Word (PSW), PCON etc. So, it required 21 unique locations for these 21 special function registers and the size of each register is of 1 byte. Some of these special function registers are bit addressable registers The program counter points to the next instruction that the CPU executes. Bit Addressable RAM 8051 has 16 bytes of RAM, which is bit addressable. It is grouped as 16, 8 byte arrays and any of the 128 bits can also be set/cleared individually. 3. Scratch PAD RAM