@Database SHOAH.s.guide
@Node MAIN "Registers"
Registers ( Major stuff now ).

Please make sure you understand this bit before moving on, it's the basis for
nearly everything else !

The 680x0 processor ( The CPU, in old Amigas you'll have a 68000, in an A1200
you'll have a 68020, etc. ) has parts to it called registers, just pretend
they're special bits of memory.

There are 8 data registers and 8 address registers, but what the hell does
that mean ? Well the data registers ( d0-d7 ) are a storage space for data
( No honestly ) and the address registers ( a0-a6 ) are for, well try and
guess. Hang on, a0-a6, where's a7 ? Well this points to something called a
stack ( A stack is just a short bit of memory for storing odds and ends
which a program needs, trust me it'll all fall into place soon ), and is
known as the stack pointer ( @{"SP" link SHOAH-SP} ) funnily enough.

Another register is the program counter ( PC ) which tells the computer
which "line number" we're on.

Yet another ( The last ) register is a 16bit ( All the others are 32bit ) one
and is called the status register ( @{"SR" link SHOAH-SR} ) and contains information about all
kinds of things.
@Endnode

@Node SHOAH-SP "Registers"
The Stack Pointer ( SP ).
	Don't mess around with this register ( a7 ). It points to a small
        list of memory ( Usually 4096 bytes, have a look at a tool icon on
	your Workbench ) in which you can store all types of information
	( Such as the SR, d0-d7, a0-a6 and the PC ).
	Just to complicate matters further, there are two types of stack
	pointer, the User Stack Pointer ( USP ) and a System Stack Pointer
	( SSP ), although you don't really have to worry about the difference
	at the moment.
@Endnode

@Node SHOAH-SR "Registers"
The Status Register ( SR ).
	This is quite complicated ( Oh good ) compared to the other registers
	, but luckily you won't have to give a toss about it's contents at
	the  moment ! ( But for the curious amongst you,...


        bit:  15  14  13  12  11  10   9   8   7   6   5   4   3   2   1   0
	--------------------------------------------------------------------
	Name:  T   -   S   -   -  I2  I1  I0   -   -   -   X   N   Z   V   C


	Bit:	Name:		But what does it do ?
	--------------------------------------------------------------------
	0	C (Carry)	Carry flag, used by maths and shift
				instruction.

	1	V (Overflow)	Like the carry flag, tells you if you've got
				a negative/postive number.

	2	Z (Zero)	Set if some operation results in 0.

	3	N (Negative)	This bit is set if an operation results in a
				negative value.

	4	X (Extended)	Also like the carry flag.

	5	-		Not Used ( On 68000 ).

	6	-		Not Used ( On 68000 ).

	7	-		Not Used ( On 68000 ).

	8	I0		The following three bits are for masking the

	9	I1		various interrupt levels, between 0 - 7.

	10	I2

	11	-		Not Used ( On 68000 ).

	12	-		Not Used ( On 68000 ).

	13	S (Supervisor)	If 0 then the processor runs in User mode, if
				1 then run in Supervisor mode.

	14	-		Not Used ( On 68000 ).

	15	T (Trace)	This bit is for single stepping the code, ie
				for debugging.
	--------------------------------------------------------------------

	so you happy now ? ).
@Endnode