DCPU-16 Assembler and Simulator
This ia an implementation of an assember and simulator for the DCPU-16 from 0x10c.
A specification of the CPU can be found at: http://0x10c.com/doc/dcpu-16.txt
A number un unclear or undefined issues remain. Here is the list of things I tripped over during development.
Harvard or von Neumann Architechture
There are two different CPU common architechtures, the basic differences relate to how the handle data and the program.
The Harvard architechture has two memory areas, the instruction memory and the data memory. As a result you have the full ammount of RAM for program data, but on the other hand it is impossible or hard for the CPU to change the running program.
The von Neumann architecture uses one block of RAM and thus intermixes instructions and data. This has the advantage that operating systems can quite flexibly change the program data, such as load new ones and remove old ones. The downside is that when writing the program you need to know where free memory is. The commonly used PC architechture IA-32 and IA-64 is a von Neumann architechture.
From the example program and the documentation, I deduced that the DCPU-16 is a von Neumann architechture. But that might be wrong.
It is unclear what the CPU guarantees for an initial state. Is every thing zero or undefined. The only obvious things are that PC is zero and SP is 0xFFFF.
What rules are applied when registers oveflow? It is somehow hinted that the normal registers will set O. But it seams that O is bit register and now a word register. How is overflow with MUL handled? Especially how is overflow on PC and SP handled?
I assumed that O is a bit register and the modulo rule applies. This may result though in data loss when you MUL two large values.
In my current implementation PC wraps around, but is will walk the entiere program memory first.
SP will truncate at the boundaries. It may make sense to trap when SP overflows thouhg.
RAM Access out of Range
What happens if RAM is accessed outside of the valid range? Does the CPU trap or is the value silently wraped?
I implemented a CPU trap for that.
The NOOP instruction is not defined. I simply assumed NOOP to match 0x0, so that the CPU does not trap when going over empy intructions.
Traps and Interupts
Is there a way to handle traps and interupts programatically? Currently nothing of this sort is defined, so I did not program anything.
Copyright (C) 2012 Sean "rioki" Farrell Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.