Optimised intoXX functions (#19)

* Optimised intoXX functions

Rewrote intoXX functions to mainly rely on intoHL, as the HL instructions are smaller and faster. Also removed some redundant push and pop instructions. I edited the given unit tests to test these, and they seem to work as expected.

* Doesn't use self-modifying code

The number of bytes is the same as my previous attempt, with 11 more cycles in intoHL, so although I don't feel as clever this time it's still a good optimisation. I found an equivalent method for intoDE, however relying on intoHL still allows for  `ex (sp), hl` to be used in intoIX, which is smaller and faster.

* Update core.asm

* Tried harder to follow coding convention

Added tabs between mnemonics and operands, and replaced a new line I accidentally removed.

kernel/core.asm

@@ -27,34 +27,29 @@ addDE:
 noop:				; piggy backing on the first "ret" we have
 	ret
 
-; copy (DE) into DE, little endian style (addresses in z80 are always have
-; their LSB before their MSB)
-intoDE:
-	push	af
-	ld	a, (de)
-	inc	de
-	ex	af, af'
-	ld	a, (de)
-	ld	d, a
-	ex	af, af'
-	ld	e, a
-	pop	af
-	ret
-
+; copy (HL) into DE, then exchange the two, utilising the optimised HL instructions. 
+; ld must be done little endian, so least significant byte first.
 intoHL:
 	push 	de
-	ex	de, hl
-	call	intoDE
+	ld 	e, (hl)
+	inc 	hl
+	ld 	d, (hl)
 	ex 	de, hl
 	pop 	de
 	ret
 
+intoDE:
+	ex 	de, hl
+	call 	intoHL
+	ex 	de, hl		; de preserved by intoHL, so no push/pop needed
+	ret
+
 intoIX:
-	push	de
-	push	ix \ pop de
-	call	intoDE
-	push	de \ pop ix
-	pop	de
+	push 	ix
+	ex 	(sp), hl	;swap hl with ix, on the stack
+	call 	intoHL
+	ex 	(sp), hl	;restore hl from stack
+	pop 	ix
 	ret
 
 ; add the value of A into HL