| author |
Steve Losh <steve@stevelosh.com> |
| date |
Sun, 25 Dec 2016 11:34:06 -0500 |
| parents |
ef37b9f3e398 |
| children |
(none) |
+++
title = "CHIP-8 in Common Lisp: Debugging Infrastructure"
snip = "Let's figure out what the hell is going on."
date = 2016-12-31T14:50:00Z
draft = true
+++
In the previous posts we looked at how to emulate a [CHIP-8][] CPU with Common
Lisp. After adding a screen, input, and sound the core of the emulator is
essentially complete.
I've been guiding you through the code step by step and it might look pretty
simple, but that's only because I went down all the dead ends myself first. In
practice, when you're writing an emulator for a system you'll need a way to
debug the execution of code, so let's look at how to add some debugging
capabilities to our simple CHIP-8 emulator.
The full series of posts so far:
1. [CHIP-8 in Common Lisp: The CPU](http://stevelosh.com/blog/2016/12/chip8-cpu/)
2. [CHIP-8 in Common Lisp: Graphics](http://stevelosh.com/blog/2016/12/chip8-graphics/)
3. [CHIP-8 in Common Lisp: Input](http://stevelosh.com/blog/2016/12/chip8-input/)
The full emulator source is on [BitBucket][] and [GitHub][].
[CHIP-8]: https://en.wikipedia.org/wiki/CHIP-8
[BitBucket]: https://bitbucket.org/sjl/cl-chip8
[GitHub]: https://github.com/sjl/cl-chip8
<div id="toc"></div>
## Disassembling
The first thing we'll need is a way to take an instruction like `#x8055` and
turn it into something we can read. The easiest way to do this seemed to be to
copy the dispatch loop from the CPU emulator and turn it into a disassembly
function:
```lisp
(defun disassemble-instruction (instruction)
(flet ((v (n) (symb 'v (format nil "~X" n))))
(let ((_x__ (ldb (byte 4 8) instruction))
(__x_ (ldb (byte 4 4) instruction))
(___x (ldb (byte 4 0) instruction))
(__xx (ldb (byte 8 0) instruction))
(_xxx (ldb (byte 12 0) instruction)))
(case (logand #xF000 instruction)
(#x0000 (case instruction
(#x00E0 '(cls))
(#x00EE '(ret))))
(#x1000 `(jp ,_xxx))
(#x2000 `(call ,_xxx))
(#x3000 `(se ,(v _x__) ,__xx))
(#x4000 `(sne ,(v _x__) ,__xx))
(#x5000 (case (logand #x000F instruction)
(#x0 `(se ,(v _x__) ,(v __x_)))))
(#x6000 `(ld ,(v _x__) ,__xx))
(#x7000 `(add ,(v _x__) ,__xx))
(#x8000 (case (logand #x000F instruction)
(#x0 `(ld ,(v _x__) ,(v __x_)))
(#x1 `(or ,(v _x__) ,(v __x_)))
(#x2 `(and ,(v _x__) ,(v __x_)))
(#x3 `(xor ,(v _x__) ,(v __x_)))
(#x4 `(add ,(v _x__) ,(v __x_)))
(#x5 `(sub ,(v _x__) ,(v __x_)))
(#x6 `(shr ,(v _x__) ,(v __x_)))
(#x7 `(subn ,(v _x__) ,(v __x_)))
(#xE `(shl ,(v _x__) ,(v __x_)))))
(#x9000 (case (logand #x000F instruction)
(#x0 `(sne ,(v _x__) ,(v __x_)))))
(#xA000 `(ld i ,_xxx))
(#xB000 `(jp ,(v 0) ,_xxx))
(#xC000 `(rnd ,(v _x__) ,__xx))
(#xD000 `(drw ,(v _x__) ,(v __x_) ,___x))
(#xE000 (case (logand #x00FF instruction)
(#x9E `(skp ,(v _x__)))
(#xA1 `(sknp ,(v _x__)))))
(#xF000 (case (logand #x00FF instruction)
(#x07 `(ld ,(v _x__) dt))
(#x0A `(ld ,(v _x__) k))
(#x15 `(ld dt ,(v _x__)))
(#x18 `(ld st ,(v _x__)))
(#x1E `(add i ,(v _x__)))
(#x29 `(ld f ,(v _x__)))
(#x33 `(ld b ,(v _x__)))
(#x55 `(ld (mem i) ,_x__))
(#x65 `(ld ,_x__ (mem i)))))))))
```
There are a lot of other ways we could have done this, like making a proper
parser or adding functionality to `define-opcode`, but since there's not that
many instructions I think this is pretty reasonable. Now we can pass in a raw,
two-byte instruction and get out something readable:
```
[SBCL] CHIP8> (disassemble-instruction #x8055)
(SUB V0 V5)
[SBCL] CHIP8> (disassemble-instruction #x4077)
(SNE V0 119)
```
Disassembling a single instruction will be useful, but it would also be nice to
disassemble an entire ROM at once to see what its code looks like. Let's make
a little helper function to handle that:
```lisp
(defun dump-disassembly (array &optional (start 0) (end (length array)))
(iterate
(for i :from start :below end :by 2)
(print-disassembled-instruction array i)
(sleep 0.001)))
```
The `sleep` is there because Neovim's terminal seems to shit the bed if you dump
too much text at it at once. Computers are garbage.
Other that than, `dump-disassembly` is pretty straightforward: just iterate
through the array of instructions two bytes at a time and print the information.
Let's look at the printing function now:
```lisp
(defun print-disassembled-instruction (array index)
(destructuring-bind (address instruction disassembly)
(instruction-information array index)
(let ((*print-base* 16))
(format t "~3,'0X: ~4,'0X ~24A~%"
address
instruction
(or disassembly "")))))
```
Once again we'll delegate to a helper function.
`print-disassembled-instruction` just handles the string formatting to dump an
instruction to the screen. Running it for a single instruction would print
something like:
```
Address Disassembly
| |
v v
200: 8055 (SUB V0 V5)
^
|
Raw instruction
```
The helper function `instruction-information` is simple, but we'll be using it
in the future for something else, so it's nice to have:
```lisp
(defun instruction-information (array index)
(let ((instruction (retrieve-instruction array index)))
(list index
instruction
(disassemble-instruction instruction))))
```
`retrieve-instruction` is simple (for now):
```lisp
(defun retrieve-instruction (array index)
(cat-bytes (aref array index)
(aref array (1+ index))))
```
These functions *could* be combined into a single, bigger function, but I'm
a strong believer in having each function do exactly one thing only. And as
we'll see, each of these "simple" tasks is going to get more complicated in the
real world.
```lisp
```
```lisp
```
```lisp
```
```lisp
```
```lisp
```
```lisp
```
```lisp
```
```lisp
```
```lisp
```