mvbg

VM_PUSH_CONST

VM_PUSH_CONST is a fundamental GBVM instruction used to push an immediate (constant) value onto the top of the VM stack.

Purpose: The GBVM is a stack-based virtual machine, and many operations require values to be present on the stack before they can be executed. VM_PUSH_CONST is essential for:

• Providing literal values: Introducing fixed numerical values (e.g., 5, 100, -1) directly into calculations or comparisons.
• Setting parameters: Passing constant arguments to functions or routines.
• Initializing variables: Providing initial values for variables before they are stored.

This instruction is one of the most basic ways to get data onto the stack for subsequent processing by other GBVM instructions, especially within RPN (Reverse Polish Notation) expressions.

Syntax:

VM_PUSH_CONST VAL

• VAL: The immediate constant value to be pushed onto the stack. This can be a positive or negative integer.

Usage Example: Performing a Simple Calculation with Constants

Imagine you want to calculate 5 + 3 and store the result in a variable VAR_RESULT. You would push the constants onto the stack and then use an RPN operation.

; In your script:

; Variable to store the result
VAR_RESULT:
  .R_INT8 0 ; Initialize to 0

; Push the constant values onto the stack
VM_PUSH_CONST 5
VM_PUSH_CONST 3

; Perform the addition using RPN
VM_RPN
  .R_OPERATOR + ; Add the two values (5 and 3) currently on the stack
  .R_INT8       ; Specify the result as an 8-bit integer
VM_STOP

; The result (8) is now on top of the stack.
; Store the result into VAR_RESULT
VM_SET_INT8 VAR_RESULT, 0 ; The 0 here is a placeholder, as VM_SET_INT8 takes the value from stack

; ... continue script ...

In this example:

1. VM_PUSH_CONST 5 places the value 5 on the stack.
2. VM_PUSH_CONST 3 places the value 3 on top of 5.
3. The VM_RPN block then uses these values. .R_OPERATOR + takes the top two values (3 and 5), adds them, and pushes the result (8) back onto the stack.
4. VM_SET_INT8 VAR_RESULT, 0 (the 0 is a placeholder for the value that will be taken from the stack) then takes the 8 from the stack and stores it into VAR_RESULT.

Analogy to other programming languages: This is analogous to directly using a literal number in an expression:

• C/C++: int result = 5 + 3;
• Python: result = 5 + 3

In assembly language, it’s like an LDI (Load Immediate) instruction that puts a constant value into a register or onto the stack. It’s the most direct way to introduce fixed data into your script’s execution flow.