Crystal Castles Assembler - Stacks

This is a continuing look at some of the features of the 6502 assembler Atari used for arcade games like Crystal Castles. One of the unique features, one I haven't seen in any other 6502 assembler, is the ability to define assembler level stacks. These have nothing to do with the 6502 stack, but instead are used to control the assembly process. 

You start by defining a stack using this pseudo op:

.DEFSTACK REGSAV, 4

There are two parameters. The first, in this case "REGSAV", is the name you want to give to the stack. You will use this to interact with the stack.

The second parameters, 4, is the size of the stack, so this stack will be able to hold up to 4 values.

To put a value on the stack you use the .PUSH pseudo op:

.PUSH REGSAV,1

This will push the value 1 onto the stack named REGSAV. The second parameter can also be an expression. I don't know if the original assembler allowed text strings to be pushed onto the stack but my version of the assembler only allows numbers to be pushed onto the stack. 

To get a value off the stack you use the .POP pseudo op:

.POP REGSAV,S1

This will pull the top value off the stack named REGSAV and put it into variable S1. The second parameter here must be a variable, not an expression.

You can use the .GETPOINTER pseudo op to get the stack pointer for a specific stack:

.GETPOINTER REGSAV,P1

This will put the stack pointer for stack REGSAV into variable P1. The stack pointer works from high to low, so if you declare a stack with the size 4, the initial pointer will be 4. If the pointer is 0 it means that stack is full.

Since I can only go by the source code I don't know what happens if you try to push a value onto a full stack or pull a value from an empty one. My version of the assembler currently doesn't handle these scenarios.

In a later post I will show how this features is used in the Crystal Castles source.