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David Schmenk 2014-05-15 21:22:44 -07:00
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@ -13,7 +13,7 @@ All three systems were implemented using stack architecture. Pascal and Java we
PLASMA takes an approach that uses the best of all the above implementations to create a unique, powerful and efficient platform for developing new applications on the Apple II. One goal was to create a very small VM runtime, bytecode interpreter, and module loader. The decision was made early on to implement a stack based architecture duplicating the approach taken by FORTH. Space in the zero page would be assigned to a 16 bit, 16 element evaluation stack, indexed by the X register.
A simple compiler was written so that higher level constructs could be used and global/local variables would hold values instead of using clever stack manipulation. Function/procedure frames would allow for local variables, but with a limitation - the frame could be no larger than 256 bytes. By enforcing this limitation, the function frame could easily be accessed through a frame pointer value in zero page, indexed by the Y register. The call stack uses the 6502's hardware stack resulting in the same 256 byte limitation imposed by the hardware. However, this limitation could be lifted by extending the call sequence to save and restore the return address in the function frame. This was not done initially for performance reasons and simplicity of implementation. Even with these limitations, recursive functions are efficiently implemented.
A simple compiler was written so that higher level constructs could be used and global/local variables would hold values instead of using clever stack manipulation. Function/procedure frames would allow for local variables, but with a limitation - the frame could be no larger than 256 bytes. By enforcing this limitation, the function frame could easily be accessed through a frame pointer value in zero page, indexed by the Y register. The call stack uses the 6502's hardware stack resulting in the same 256 byte limitation imposed by the hardware. However, this limitation could be lifted by extending the call sequence to save and restore the return address in the function frame. This was not done initially for performance reasons and simplicity of implementation. Even with these limitations, recursive functions can be effectively implemented.
One of the goals of PLASMA was to allow for intermixing of functions implemented as bytecode, or native code. Taking a page from the FORTH play book, a function call is implemented as a native subroutine call to an address. If the function is in bytecode, the first thing it does is call back into the interpreter to execute the following bytecode. Function call parameters are pushed onto the evaluation stack in order they are written. The first operation inside of the function call is to pull the parameters off the evaluation stack and put them in local frame storage. Function callers and callees must agree on the number of parameters to avoid stack underflow/overflow. All functions return a value on the evaluation stack regardless of it being used or not.