mvbg

VM_SET_INDIRECT

VM_SET_INDIRECT is a GBVM instruction used to assign a value from one variable to another variable that is addressed indirectly, using an index stored in a third variable.

Purpose: This instruction is the counterpart to VM_GET_INDIRECT. While VM_GET_INDIRECT retrieves a value from an indirectly addressed variable, VM_SET_INDIRECT allows you to write a value to such a variable. This is crucial for:

• Modifying array elements dynamically: Changing the content of a specific inventory slot, a tile in a map, or a character’s stat in a list, where the target element’s index is determined at runtime.
• Flexible data manipulation: Updating different variables based on a calculated or stored index, enabling more dynamic and data-driven game logic.

Syntax:

VM_SET_INDIRECT IDXA, IDXB

• IDXA: The variable that contains the index (or offset) of the target variable. This variable tells the GBVM where to write the value.
• IDXB: The source variable that contains the value to be assigned. This variable holds the data that will be written.

Usage Example: Updating an Inventory Slot

Imagine you have an inventory system where VAR_INVENTORY_SLOT_0, VAR_INVENTORY_SLOT_1, etc., represent inventory slots. You have a variable VAR_TARGET_SLOT that stores the index of the slot you want to modify, and VAR_NEW_ITEM_ID holds the ID of the item you want to place in that slot.

; In your script:

; Inventory slot variables (conceptual, assumed to be contiguous)
VAR_INVENTORY_SLOT_0:
  .R_INT8 0  ; Empty
VAR_INVENTORY_SLOT_1:
  .R_INT8 10 ; Sword ID
VAR_INVENTORY_SLOT_2:
  .R_INT8 25 ; Potion ID

; Variable to hold the target slot index
VAR_TARGET_SLOT:
  .R_INT8 0 ; Initialize to slot 0

; Variable to hold the new item ID to place
VAR_NEW_ITEM_ID:
  .R_INT8 0 ; Initialize to 0

; Scenario: Player picks up a new item (e.g., a Shield, ID 50) and wants to put it in slot 0.
VM_SET_INT8 VAR_TARGET_SLOT, 0    ; Set target slot to 0
VM_SET_INT8 VAR_NEW_ITEM_ID, 50   ; Set new item ID to 50 (Shield)

; Assign the value from VAR_NEW_ITEM_ID to the inventory slot pointed to by VAR_TARGET_SLOT
VM_SET_INDIRECT VAR_TARGET_SLOT, VAR_NEW_ITEM_ID

; Now, VAR_INVENTORY_SLOT_0 will contain 50 (Shield ID).

; Scenario: Player wants to replace the Sword (slot 1) with an empty slot (ID 0).
VM_SET_INT8 VAR_TARGET_SLOT, 1    ; Set target slot to 1
VM_SET_INT8 VAR_NEW_ITEM_ID, 0    ; Set new item ID to 0 (empty)

VM_SET_INDIRECT VAR_TARGET_SLOT, VAR_NEW_ITEM_ID

; Now, VAR_INVENTORY_SLOT_1 will contain 0 (empty).

In this example, VM_SET_INDIRECT uses the value in VAR_TARGET_SLOT (e.g., 0 or 1) to determine which global variable (VAR_INVENTORY_SLOT_0 or VAR_INVENTORY_SLOT_1) to write to. It then takes the value from VAR_NEW_ITEM_ID and places it into that indirectly addressed inventory slot.

Analogy to other programming languages: This is analogous to assigning a value to an element of an array using a variable as the index:

• C/C++: inventory_array[target_slot] = new_item_id;
• Python: inventory_list[target_slot] = new_item_id

It provides a way to dynamically update the content of a variable whose identity is itself stored in another variable, enabling powerful indirect addressing capabilities for writing data.