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Relays, Memory Areas and Constants
1 - 58
1.3.9
Index Registers (IX, IY) (for FP0, FP
−
e)
Function of index registers (IX, IY)
Index registers are used to indirectly specify constants and memory area addresses.
Two 16
−
bit registers are available, IX and IY. Changing addresses and constants using
a value in an index register is called “index modification”.
With the FP0, FP
−
e, index modification is possible only with regard to operands of
high
−
level instructions.
Modifying an address
Address = Base a Value in IX or IY (K constant)
Example:
Modifying DT11
IXDT11
Base address
IX value
Target address
11
+
K0
=
DT11
11
+
K10
=
DT21
11
+
K
−
10 =
DT1
Modifying a constant
Constant = Base value + Value in IX or IY
Example 1: Modifying K100
IXK100
Base value
IX value
Constant
K100
+
K0
=
K100
K100
+
K10
=
K110
K100
+
K
−
10 =
K90
Example 2: Modifying H10
IXH10
Base value
IX value
Constant
H10
+
HA
=
H1A
H10
+
H10
=
H20
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