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+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
+```