Zilog Z8F0130 - Manual (Page 198)

eZ8
™
CPU Core
User Manual
UM012820-0810 LDX Instruction
183
If the high byte of the source or destination address is EEh ( 11101110b ),
a working register is inferred. For example, the operand EE3h selects
Working Register R3. The full 12-bit address is provided by {RP[3:0],
RP[7:4], 3h} .
To access registers on Page Eh (addresses E00h to EFFh ), set the Page
Pointer, RP[3:0], to Eh and set the Working Group Pointer, RP[7:4] , to
the preferred Working Group.
.ER Notation
The .ER notation is used with Op Codes 87h and 97h , to distinguish
between these two cases, which might otherwise be ambiguous. The
potential ambiguity arises because in both these Op Codes, the two oper-
ands are both indirect registers, to be prefixed with the @ symbol. How-
ever, one of these is to be treated as an indirect Register Pair, which are
combined to give a 16-bit address, while the other is to be treated as a sin-
gle register which yields an 8-bit address. Without some way of distin-
guishing between the two cases, it would be impossible for the assembler
to determine whether the user’s purpose was to treat the destination oper-
and as a Register Pair and the source as a single register, or vice versa.
The
.ER notation resolves this ambiguity. The operand which is prefaced
with .ER is taken to be a Register Pair, and the other is treated as a single
register. For detailed information, see the examples provided for Op
Codes 87 and 97 in the
Sample Usage section.
Sample Usage
If Register 702h contains the value B3h , the following statement loads
the value B3h into Working Register R1:
LDX R1, 702h
Object Code: 84 17 02
The contents of register 702h are not affected.