While having all instructions be the same length can offer benefits in certain architectures, it is not considered a good idea for stack machines due to the nature of their instruction set design and execution model. Here are a few reasons why:

• Variable Operand Count: Stack machines typically use a stack data structure to perform operations. Instead of explicit operands in the instruction itself, stack machines rely on pushing and popping values onto and from the stack. The number of operands required for an instruction can vary based on the operation being performed. Forcing a fixed instruction length would limit the flexibility of stack machine instructions and make it challenging to handle variable operand counts efficiently.

• Stack-based Operations: Stack machines heavily rely on stack operations such as pushing values, popping values, and manipulating the top elements of the stack. These operations are fundamental to the execution of programs on a stack machine. With a fixed instruction length, accommodating stack operations along with the necessary information for operands and addressing modes would be difficult and could lead to overly long instructions.

• Code Density: Stack machines aim for compact code representation to minimize memory usage and improve instruction cache utilization. By allowing instructions to vary in length based on the specific requirements of each operation, stack machines can achieve better code density. Instructions that require fewer operands or have simpler operations can be represented with shorter instruction encodings, resulting in more efficient use of memory.

• Simplified Instruction Fetch: Stack machines often have a simplified instruction fetch mechanism, where each instruction is fetched sequentially from memory. By allowing variable instruction lengths, the fetching mechanism can be more straightforward and more efficient. Fixed instruction lengths would require more complex and potentially slower instruction fetching mechanisms, as they would need to account for variable-length instructions.

• Optimization Opportunities: The variable-length instruction format in stack machines allows for optimization opportunities. The compiler or interpreter can analyze the code and choose appropriate instruction sizes based on the requirements of each operation. This flexibility can enable better instruction scheduling, packing more operations into limited memory, and potentially improving performance.

In summary, variable-length instructions in stack machines offer flexibility, compact code representation, simplified instruction fetch, and optimization opportunities. These advantages align well with the stack-based execution model and the nature of stack machine operations. Fixed instruction lengths would introduce unnecessary complexity and limitations, making it less suitable for efficient stack machine architectures.