PC is a 16-bit register. It contains a memory address. PC contains that very memory address from where the next instruction is to be fetched for execution. Suppose the PC contents are 8000H, then it means that the 8085 Desires to fetch the instruction Byte at 8000H. After fetching the Byte at 8000H, the PC is automatically incremented by 1. This way 8085 becomes ready to fetch the next Byte of the instruction (in case instruction fetch is incomplete), or fetch the next opcode (in case instruction fetch is over).So in this example, first of all PC is loaded with ... Read More

Reading from the StackLet us consider that SP contents the address FC78H, and we want to read information from a stack location. In this case, we are not interested in reading from a location whose address is less than the memory address present in SP. This is because 8085 interprets them as useless information. For example, there is no point in reading useless information from memory location FC75H.SP interprets that, memory locations FC78H, FC79H, ..., FFFFH are all interpreted by 8085 to have useful information. To read from Stack, the instruction is POP in 8085 microprocessor instruction set. Here we ... Read More

The stack is a LIFO (last in, first out) data structure implemented in the RAM area and is used to store addresses and data when the microprocessor branches to a subroutine. Then the return address used to get pushed on this stack. Also to swap values of two registers and register pairs we use the stack as well.In the programmer‘s view of 8085, only the general purpose registers A, B, C, D, E, H, and L, and the Flags registers were discussed so far. But in the complete programmer’s view of 8085, there are two more special purpose registers, each ... Read More

Many a time, we are required to represent decimal numbers in a computer, and perform arithmetic on these numbers. For example, we may be required to total the marks a student has obtained in five different subjects, where obviously, the marks are awarded in decimal notation.For this purpose, the BCD code is extensively used. In BCD notation, 4 bits are used to code a digit, and so two digits of information are stored in a Byte. For example, decimal 45 is represented in BCD as of 0100 0101. Codes 10 to 15 i.e. 1010, 1011, 1100, 1101, 1110, and 1111 ... Read More

In a digital computer, everything is represented using 0s and 1s only. For example, instruction will have a code using only 0s and 1s. Data is also represented using 0s and 1s. Data can be of different types like unsigned numbers, signed numbers, floating point numbers, binary coded decimal (BCD) numbers, etc. Thus, a series of 0s and 1s will acquire a value based on the interpretation. For decimal addition, we are having a very important and common instruction DAD. Let us discuss more on that instruction now.In spite of the fact that 8085 is an 8-bit microprocessor, but there ... Read More

In the subtraction of two numbers, 8085 imposes the restriction that Accumulator will have one of the operands from which the other operand specified by one of the following will be subtracted.—Contents of an 8-bit register;—Contents of memory location pointed by HL pair;—Eight-bit immediate data.In 8085 Instruction, SUB is a mnemonic that stands for ‘SUBtract contents of R from Accumulator’. Here R stands for any of the following registers, or memory location M pointed by HL pair.R = A, B, C, D, E, H, L, or MMnemonics, Operand Opcode (in HEX)BytesSUB A971SUB B901SUB C911SUB D921SUB E931SUB H941SUB L951SUB M961In this instruction ... Read More

In 8085 microprocessor, the flags register can have a total of eight flags. Thus a flag can be represented by 1 bit of information. But only five flags are implemented in 8085. And they are:Carry flag (Cy), Auxiliary carry flag (AC), Sign flag (S), Parity flag (P), andZero flag (Z).The respective position of these flag bits in flag register has been show the below figure. The positions marked by “x” are to be considered as don't care bits in the flags register. The user is not required to memorize the positions of these flags in the flags register.Fig. Flags registerNow ... Read More

In the ALS-SDA-85M kit, we have 74138 EPROM of minimum size 16K x 8 and size of RAM IS 2K x 8, there are empty sockets for the EPROM to get expanded. The selection of these four chips is done by the integrated circuit 74138. Every time the selection of 74138 is done when IO/M* turns to 0, the election of 27138 is done when A15A14 turns to 0. 27138 have the lowest address range likeLowest address: 00 00000000000000 is 0000H.Highest address: 00 11111111111111 is 3FFFH.Whereas in 74138 the address ranges are as follows -Lowest address: 11 xxx 00000000000Highest Address: 11 xxx 11111111111.The ... Read More

Let’s consider that the EPROMs we have are having the starting addresses as 4000H, 4400H, …, 5C00H. 4000H in binary is 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0. In such a case, the 74138 has to be selected for the condition A15 A14 A13 = 0 1 0. This can be achieved by the connection shown in below figure which uses two invert gates.The procedure of selecting the chip74138 is denoted by the addresses A15 A14 A13 = 0 1 0 which can be implemented by any gates. We have taken ... Read More

Let’s take an Integrated Circuit decoder. We take the popular 3 to 8 decoder Integrated Circuit 74138. The Integrated Circuit is of 16 pins.We have three input pins which are actively in high state and are classified as I2, I1 and I0. The outputs are actively in low state and are eight in number and are classified as O7*, O6*, …, O0*. A power supply of +5 V DC is needed by the chip and is Grounded.                                                ... Read More