Tasks:
1.Define PSW bit-fields for a custom processor.
2. Simulate interactions between ALU and PSW.
PSW (Program Status Word) is a special-purpose register in a processor that holds information about the current state of the processor and controls the execution flow. It typically contains flags that indicate the status of various operations and settings that affect the processor's behavior.
Status Flags:
Zero Flag (Z): Indicates if the result of an operation is zero.
Carry Flag (C): Indicates if there is a carry out of the most significant bit (used in arithmetic operations).
Sign Flag (S): Indicates the sign (positive or negative) of the result.
Overflow Flag (O): Indicates if an arithmetic overflow occurred.
Parity Flag (P): Indicates if the result has an even or odd number of 1s.
Control Flags:
These flags control the operation of the processor, such as enabling interrupts or determining the execution mode.
Program Flow Control:
The PSW helps manage program flow, making decisions on whether to branch, interrupt handling, or handle exceptions based on the condition flags.
Simulate interactions between ALU and PSW.
The ALU performs arithmetic and logic operations like addition, subtraction, AND, OR, etc. After each operation, the ALU provides results and status flags (like Zero, Carry, Overflow, and Sign) that need to be updated in the PSW.
The PSW holds the state of these flags:
Zero Flag (Z): Set when the result of the ALU operation is zero.
Carry Flag (C): Set when there’s a carry-out from the most significant bit in addition or a borrow in subtraction.
Overflow Flag (V): Set when the result exceeds the range of the register in a signed operation.
Sign Flag (S): Indicates if the result is negative in signed operations.
Step 1: ALU Operation
The ALU receives two operands, performs an operation (e.g., addition), and produces a result.
Step 2: PSW Update
Based on the result of the operation, the ALU will set or clear the appropriate flags in the PSW.
For example:
If the result of the operation is zero, the Zero flag (Z) in the PSW is set.
If there's a carry, the Carry flag (C) is set.
If the result causes an overflow in signed numbers, the Overflow flag (V) is set.
If the result is negative, the Sign flag (S) is set.
Step 3: Use of Flags in Program Flow
After the ALU updates the PSW, the processor can use these flags to control branching or decision-making, such as in conditional jumps (e.g., jump if zero, jump if overflow, etc.).
Initial Values:
Operand 1: 15 (in binary: 00001111)
Operand 2: 5 (in binary: 00000101)
Operation: Add Operand 1 and Operand 2 using the ALU.
ALU Result: 15 + 5 = 20 (in binary: 00010100)
PSW Update:
Zero Flag (Z): The result is not zero, so the Zero flag remains cleared.
Carry Flag (C): No carry occurs, so the Carry flag remains cleared.
Overflow Flag (V): No overflow occurs, so the Overflow flag remains cleared.
Sign Flag (S): The result is positive, so the Sign flag remains cleared.