Inside the 8085, instructions are really stored like binary numbers, not a very good manner to look at them and very difficult to decipher. An assembler is a program that permits you to write instructions in, more or less, English form, much more simply read and understood, and after that converted or assembled into hex numbers and at last into binary numbers.
The program is written with a text editor (NOTEPAD or similar), saved like an ASM file, and then assembled by the assembler (TASM or MASM or similar) program. The end result is an OBJ file you download to the 8085. Here is an instance of the problem of adding 2 plus 2 :
mvi A,2; move 2 into the A register
mvi B,2; move 2 into the B register
add B; add reg. B to reg. A, store result in reg. A
The first line moves a 2 into a register A. The second line moves a 2 into a register B. it is all the data we require for the program. The third line adds up the accumulator with register B and holds the result back into the accumulator, destroying the 2 that were originally in it. The accumulator contains a 4 in it now and B still contains a 2 in it. In the program above all text after the ';' are treated as comments, and not executed. It is a very significant habit to acquire.
Assembly language follows some rules that we shall describe as they come up. With most of the instructions, especially those including data transfer, the instruction is first, followed by at least 1 space, after that the destination followed through a comma, and then the source. The destination is where the result of the instruction shall end up and the source is where the data is coming from.
Next we shall read a switch, and light an LED if the switch is pressed. This happens quite frequently in your lab experiments. Bit 0 of Port 0 shall be the switch. While the switch is closed or pressed, bit 0 will be a 1, and if the switch is open or not pressed, bit 0 will be a 0. Bit 0 of Port l is the LED. If bit 0 is a 0 the LED is off and if bit 0 is a 1, the LED shall be on. All the other bits of reg. A shall be ignored and assumed to be all 0's, for the sake of discussion
IN 0 ; read Port 0 into reg. A
CMP 1 ;compare reg. A with the value 1
JNZ start ; jump to start if the comparison does not yield 0
OUT 1 ; send a 1 to Port 1, turning the LED on
The first line has something new. It's called as a label. In this case it is start:. A label is a manner of telling the assembler that this line has a name that may be referred to later to get back to it. All of the labels are followed by the symbol:, that tells the assembler that it is a label. In the first line we also read the switch by reading Port and putting it into the accumulator. Register. A is the only register that may read in/send out data using ports or perform compares. Therefore, we need not write 'A' in the command it's implied.