0 Address
Duration: 2 min
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The video lecture introduces 0-Address Instructions used in stack-organized computers. The instructor explains that early computers with limited registers utilized an organized stack where the result location is implied at the top of the stack (TOS). Consequently, instructions like ADD and MUL do not require an address field, unlike PUSH and POP which need operands. To evaluate arithmetic expressions, the expression must be converted into Reverse Polish Notation (RPN). The lecture demonstrates this with the expression X = (A + B) * (C + D), showing the corresponding assembly code and stack operations. The instructor notes that while this architecture is simple, it is complex to design and has poor speed.
Chapters
0:00 – 2:00 00:00-02:00
The instructor begins by defining 0-Address Instructions, highlighting the slide text: "Early computers which do not have complex circuit like ALU use few registers which are used as organised stack." He explains that in a stack-organized computer, the result location is implied at the top of the stack, so instructions like ADD and MUL do not use an address field. He points out that PUSH and POP instructions, however, need an address field to specify the operand. The slide text states: "To evaluate arithmetic expression in a stack computer, it is necessary to Convert the expression into reverse polish notation." He emphasizes that the name "zero-address" comes from the absence of an address field.
2:00 – 2:23 02:00-02:23
The instructor focuses on the example program shown on the slide: `program shows how X = (A + B) * (C + D) will be written`. He points to `PUSH A` and `TOS <- A`, then `PUSH B` and `TOS <- B`. He explains the `ADD` instruction where `TOS <- (A + B)`. He continues with `PUSH C`, `PUSH D`, and the second `ADD` resulting in `TOS <- (C + D)`. Finally, he explains the `MUL` instruction where `TOS <- (C + D) * (A + B)` and the `POP X` instruction which stores the result in memory `M[X] <- TOS`.
The lecture effectively bridges the theoretical definition of 0-Address Instructions with a practical assembly language example. By breaking down the evaluation of X = (A + B) * (C + D), the instructor clarifies how the stack pointer (TOS) manages operands implicitly. The key takeaway is the trade-off: the simplicity of instruction format (no address fields for arithmetic) versus the complexity of design and execution speed.