Instructions are made up of a variable number of "words" and those words are made up of bits. In the case of the 8085 architecture, you can have instructions that are one, two, or three words long, and each word is 8-bits. Those bits are divided up into fields based on your instruction set. What fields there are and what those fields mean is usually contextually sensitive based on the value of one field that always has the same meaning. Typically this field is referred to as the "op code." You should read in detail at least chapter 8 of the linked pdf to get a thorough understanding.
In the case of instructions that operate on registers and store their results in a register, the source and target registers need to be identified in some fields in the instruction. In the case of an instruction that adds a constant to a register and stores the result in a register, the registers still need to be identified, but the constant also needs to be encoded in the instruction in its own field. In instruction set architectures, the term "immediate" is often used to mean a "constant value." In case of instructions that read from or write to memory, the location in memory may have to be encoded within the instruction.
That's the basic idea, hope this helps. For future reference, a good search term for questions like this is "Instruction Set Architecture" for your processor.
Edit Re: STA 4200
The STA instruction is described on page 3-61 (pg 117) of this Assembly Language Programming guide for 8080/8085 processors.The three bytes are:
- Byte 1 = OpCode (00110010)
- Byte 2 = Low Address Byte
- Byte 3 = High Address Byte
STA is the "Store Accumulator Direct" instruction, and what it does is copies the value of the Accumulator into memory at the 16-bit address composed from Bytes 2 and 3.
Correction.
I made the unfortunate dyslexic mistake of reading the three letter abbreviation as RMW when it was RWM. In the context of the block diagram of the original question I would have to agree that -
RWM means "Read - Write - Memory" as in an operational sequence.
I'll leave my original comment below as it is also a very important concept in computer architecture too.
RMW means "Read - Modify - Write".
Typically this is in the context of the computer architecture that the RWM operation is designed in such way that it is an atomic sequence. This means that a register contents can be partially changed in one operation without being interrupted by an other part of the computer before the register update is complete.
Best Answer
In (digital) electronics latching means that the signal "locked" in a certain state (zero or one) unless a clock or other control signal allows it to change.
So for the 8255A the inputs are not latched, this means that the chip will respond immediately (I'm ignoring the effect of delays which are always present) to any change at the inputs.
The 8255A's outputs are latched so that means they will only change when a certain control signal "says so". Look at the datasheet of the 8255A on the first page there's this drawing:
Note how there's a signal from the "Read Write Control Logic" (bottom left) going to all the Group ports on the right. I bet that that is the latching signal for the outputs.
In general it is convenient to have signals in digital logic only change at the transitions of the clock. Any signal which does not meet this requirement already can be latched (with the clock) and then it will meet this requirement.