Xerox 560 Reference Manual page 27

Instruction in memory:
Instruction in instruction register:
I IIII
Byte operation indexing
01
ignment:
Halfword operation indexing
01
ignment:
Word operation indexing
01
ignment:
Shift operation indexing alignment:
Doubleword operation indexing
01
ignment:
Effect i ve vi rtua I address:
Figure 5. Index Displacement AI ignment (Real and Virtual Addressing Modes)
ADDRESS MODIFICA nON EXAMPLE: INDIRECT,
INDEXED HALFWORD (VIRTUAL ADDRESSING)
Figure 7 illustrates the address modification and mapping
process for an indirectly addressed
t
indexed, halfword op-
eration. As shown, reference address 1 is the content of
the reference address field in the instruction stored in mem-
ory. The instruction is brought into the instruction register,
and if the value of the reference address field is greater
than 15, the memory map converts the 19-bit effective vir-
tual address into a 22-bit actual address. The 17 low-order
bits of the main memory location pointed to by the actual
address, labeled reference address 2, then replaces refer-
ence address 1 in the instruction register. The index register
designated by the X field of the instruction is subsequently
aligned for incrementing at the halfword-address level. The
final effective virtual address is formed by the address gen-
erator, and if the value of the reference address is greater
than 15, the effective virtual address is transformed through
the memory map into an actual address. The resultant 22-bit
actual (main memory) address, which automatically contains
a low-order 0, is then used to access the halfword to be
used as the operand for the instruction.
Note that for the real addressing mode, the modifications
required for indirect, indexed halfword operation are the
same with one exception: reference address 1 and the final
effective address are concatenated with three leading zeros
(as opposed, to being transformed by the memory map).
REAL-EXTENDED ADDRESSING
Real-extended addressing is similar to real addressing in that
a direct relationship exists between the effective virtual ad-
dress of each instruction and the actual address. The func-
tion of real-extended addressing is to foci litate operations
in a memory system larger than 128K words.
Note: Instructions and indirect addresses that involve
real-extended address calculations must themselves
reside in the first 128K words of memory (or in the
general registers), although they in turn may ulti-
mately access operands in locations beyond the first
128K words of memory.
Main Memory 21