spec.txt

hack is set up as follows:
2 16-bit registers, A and D. 
D is a data register
A can be used for data or addresses

Instruction memory stores instructions
It has addresses of 15 bits and holds 16-bit words
Instruction memory is read-only

Data memory stores data (duh)
Like instruction memory, it has 15-bit addresses and 16-bit words
Data memory is writable

Implicit things abound, oh joys
Since data memory uses 15-bit addresses, we load an address into A
And we alias the register M to refer to Memory[A]

Similarly, we use A for direct memory access to instruction memory
So if we wanted to jump to instruction memory 35,
We load 35 into A, and then do a blank GOTO 
This jumps to InstructionMemory[A]

Finally, we have the special syntax @value
value can be either a literal integer or a name
This is a load instruction; it assigns value to A
This syntax is called an A-instruction. 
The op code is 0vvv vvvv vvvv vvvv
Where vvv vvvv vvvv vvvv is the binary representation of value

C-instructions do actual computation
In general, dest=comp;jump
dest may be empty, in which case the = is omitted
jump may be empty, in which case the ; is omitted
The op code is 111a cccc ccdd djjj

a is a flag for the type of instruction
cccc cc is a code for computation
 full table in pdf
 when a=0
 	opcode	result
 	101010  0
 	111111	1
 	111010	-1
 	001100	D
 	110000	A
 	001101	!D
 	110001	!A
 	001111	-D
 	110011	-A
 	011111	D+1
 	110111	A+1
 	001110	D-1
 	110010	A-1
 	000010	D+A
 	010011	D-A
 	000111	A-D
 	000000	D&A (logical AND)
 	010101	D|A (logical OR)
 when a=1, replace all instances of 'A' with 'M'

dd d is a code for which location to store the value
 first bit on to store to A
 second bit on to store to D
 third bit on to store to M --> recall, M = Memory[A]
Any or all of these may be set

jjj is a code for the condition on which to jump to InstructionMemory[A]
 first bit on to jump if output of computation is < 0
 second bit on to jump if output of computation is = 0
 third bit on to jump if output of computation is > 0

Some predefined symbols:
Virtual registers R0 - R15 point to RAM[0] - RAM[15] 
SP points at RAM[0]
LCL points at RAM[1]
ARG points at RAM[2]
THIS points at RAM[3]
THAT points at RAM[4]

Labels are defined with (labelname)
Variables are automatically assigned a RAM location, starting at RAM[16]

I/O handling also exists, we'll get to that later