Specifications

CHAPTER 3
ADDRESSING jdODES‘
Most data in a program is structured in some way-in a table, in a stack, in
a table of addresses, op perhap$ in a small set of frequently-used variables‘
local to a limited region of a program. The PDP-11 handles these common
data structures -with addressing modes specifically designed for each kind
of access. In addition, addressing for unstructured data is general enotfgh *’
to permit direct random ac%ess to all of core. Memory is not brokeri up into
pages and fields (often awkward and wasteful of core storage).
Addressing in the PDP-11 is dohe through the general registers. PrOWems
requiring several stacks can use the general registers for stack pointers.
Those requiring many local variables can use general registers as accumu-
lators. The general registers can be used interchangeably as index ,registen
or as sequential list pointers to access tabularrdata. Address arithmetic may
be done directly in the general registers. ,
ADDRESS FIELDS
PDP-11 instruction words contain a 6-bit address field divided into two sub-
fields selecting the general register and the mode generating the effective
address.
The register subfield specifies which of the eight general registers is to be
used in the address calculation. The mode subfield indicates how this register
is to be used in determining the operand. These modes will be described
in the following paragraphs.
GENERAL REGISTER ADDRESSING-The general registers will be used .as
simple accumulators for operating on frequently-accessed variables. In this
mode, the operand is held directly in the general register. The general reg-
isters are in iast memory, (280-nanosecond cycle time) resulting in a speed
improvement for operations on these variables.
PAL-11, the PDP-11 assembler, interprets instructions of the form
OPR R
as general register operations. R has been defineb as a register name and
OPR is used to represent a general instruction mnemonic. ihe address field
for general register operations is
Operands that are pointed to -by addresses (indirect or deferred) are de-
noted to PAL-11 by the @ symbol. Thus, instructions of the form
* .
OPR @R
specify deferred register addressing and have the following address field.
,
11