Xerox 550 User Manual

INFORMATION BOUNDARIES
Basic processor instructions assume that bytes, halfwords,
and doublewords are located in main memory according to
the following boundary conventions:
1. A byte is located in bit positions 0 through 7, 8
through 15,
16 through 23, and 24 through 31 of a
word.
Doubleword
Word (even address)
Word (odd address)
Halfword
0
Halfword 1
Halfword
0 Halfword 1
Byte
0 I Byte 1
Byte
21Byte 3
Byte
0 I Byte 1 Byte 21Byte 3
2. A halfword is located in bit positions 0 through 15 and
16 through 31 of a word.
3. A doubleword is located such that bit positions 0 through
31 are contained within an even-numbered word, and
bit positions 32 through 63 are contained within the
next consecutive word (which is odd-numbered).
Figure 3 illustrates these boundaries.
Doubleword
Word (even address)
Word (odd address)
Halfword
0
Halfword 1 Halfword 0
Halfword 1
Byte
0 I Byte 1 Byte 21Byte 3
Byte
0IByte 1 Byte 21 Byte 3
Figure
3. Information Boundaries
INSTRUCTION REGISTER
processor is currently executing. The format and fields of
the two general types of instructions (memory reference and
immediate operand) are described below. Specific formats
for
each instruction are given in Chapter 3.
MEMORY REFERENCE INSTRUCTIONS
Instructions that make reference to an operand in main mem-
ory may
have the following format:
Bits
o
1-7
8-11
Description
Indirect
addressing. One level of indirect ad-
dressing is performed only if this bit position con-
tains a one.
Operation code. This 7-bit field contains the code
that designates the operation to be performed. See
the inside front and back covers for complete list-
ings of operation codes.
R field. For most instructions this 4-bit field des-
ignates one of the first 16 general registers of the
Bits
8-11
(cont. )
12-14
Descri pti on
current register block as an operand source, result
destination, or both.
X field. This 3-bit field designates one of general
registers 1-7 of the current register block as an
index register. If X contains zero, indexing wi II
not be performed; hence register 0 cannot be used
as
an index register. (See "Address Modification
Example: Indexing (Real and Virtual Addressing)",
later in this chapter for a description of the
i ndexi ng process. )
15-31 Reference address. This
17-bit field normally con-
tains the reference address of the instruction oper-
and. The reference address is translated into an
effective virtual address in accordance with the
addressing type
(real, real extended, or virtua I)
and the address modification requied (direct/
indirect or indexing). (See "Memory Reference
Addresses"
later in this chapter. )
IMMEDIATE OPERAND
INSTRUCTIONS
Immediate operand type instructions are particularly effi-
cient because the required operand is contained within the
Bas i c Processor 13

Xerox 550 Specifications

General

Brand	Xerox
Model	550
Category	Printer
Language	English