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01 января 1995 |
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Playing With The Index Registers
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By Bogie Of E-3
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NB: Some knowledge of m/c is needed for this article!!!
In this article I am going to tell you things about IX and IY
you NEVER knew. In all the manuals and m/c books that I have
seen, they state that IX and IY cannot be split up, i.e.
LD X,50
is not possible to do. But infact you CAN do this!!!
I started looking into this when ICABOD told me that he had
found out (via the internet) that there are ways of doing this.
So I decided to look further into how IX and IY work!!! All
that the IX and IY do is take the op-code for HL and prefix it
with #DD for IX and #FD for IY.
For convenience I shall call the hi byte of IX hX and the low
byte of IX X. I shall only use IX today but if you wish to use
IY then just change #DD to #FD!!!!
For a small example type this into yor assembler and PRINT USR
the address to which you assemble it to!!!
LD IX,0
LD HL,65535
DEFB #DD
LD L,50
PUSH IX
POP BC
RET
Because it looks like I don't change IX in anyway, you would
expect 0 to be printed. But infact 50 is printed!!!! This is
due to 'Prefixing' LD L,50 by #DD, which changes it into
LD X,50. Which apparantly is impossible!!!!!
It is also possible to LoaD hX and X by other registers!!!!
LD IX,0
LD DE,12345
DEFB #DD
LD H,D
DEFB #DD
LD L,E
PUSH IX
POP BC
RET
If you PRINT USR then 0 will not be Printed but infact 2345
will be printed!! This is because this is what you have really
DONE:
LD IX,0
LD DE,12345
LD hX,D
LD X,E
PUSH IX
POP BC
RET
Which your assembler will not be able to cope with because
according to Sinclair Research, is NOT possible!! There are
some LD X,r which I have yet to find out how to do, these are:
LD X,Y
LD hX,Y
LD X,hY
LD hX,hY
LD X,H
LD X,L
LD hX,H
LD hX,L
These maybe possible but not in an easy to understand way, but
if ICABOD can find the informtion on the internet again, then
maybe he can update us in the next issue of Subliminal Extacy
You can also use hX and X in logic operations, for example:
LD IX,0
DEFB #DD
LD L,69
LD L,0
LD A,69
DEFB #DD
AND L
DEFB #DD
LD L,A
PUSH IX
POP BC
RET
What this does is LoaD X,69
LoaD A,69
And X
So 69 And 69 = 69, so 69 will be printed with PRINT Usr.
I hope you can understand this article but just incase here is a
basic run down of this article:
DEFB #DD LD H,n = LD hX,n
DEFB #DD LD H,r = LD hX,r
DEFB #DD LD L,n = LD X,n
DEFB #DD LD L,r = LD X,r
DEFB #DD AND H = AND hX
DEFB #DD AND L = AND X
DEFB #DD OR H = OR hX
DEFB #DD OR L = OR X
DEFB #DD XOR H = XOR hX
DEFB #DD XOR L = XOR X
DEFB #DD INC H = INC hX
DEFB #DD INC L = INC X
DEFB #DD DEC H = DEC hX
DEFB #DD DEC L = DEC X
That's all for now, happy coding. Bogie of E-3!
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