@c register-memory.botex
@c
@c 24-May-88, James Rauen

@chapter Register Memory

This chapter describes the organization of the K processor's
general-purpose registers.


@section Register Frames

The processor's general purpose registers are organized into 256
@i[register frames], each of which contains sixteen 33-bit registers.
The contents of each register are arranged according to the formats
outlined in the Storage Conventions chapter.

Sixteen of the register frames (those numbered from #x00 to #x0F) are
reserved as @i[global frames].  These frames are not affected by the
various function call operations performed by the call hardware.
The other 240 @i[local frames] provide fast storage for local variables,
function arguments, and returned values.


@section Open, Active, and Return Frame Registers

At any particular time, the processor has access to three of the local
frames.  These three frames are determined by the contents of three
special-purpose 8-bit registers: the Open Frame, Active Frame, and
Return Frame registers.  The registers in these three frames may be used
as sources or destinations for any ALU operation.  (See the Instruction
Set chapter).

Normally, the contents of the Open Frame, Active Frame, and Return Frame
registers are manipulated only by the call hardware.  It is possible to
directly read and write their contents by accessing functional
destination 1001010 (OPEN-ACTIVE-RETURN).  The only code which should
write this destination is the system software which sets up, dumps, and
restores the call hardware.

The assembler provides a shorthand for referring to registers in the
current Open, Active, and Return frames.  The sixteen registers in the
frame addressed by the Open Frame register are labeled O0, O1, and so on
up to O15.  Likewise, the sixteen registers in the current active frame
are labeled A0 through A15.  The sixteen registers in the current return
frame are labeled R0 through R15.


@section Global Registers

The processor always has access to the sixteen global frames.  Each instruction
contains a Global Frame Number field which designates which global frame
the instruction wants to use.  The registers in this global frame may
be used as sources or destinations for any ALU operation.


@section Differences in Chip Set

Due to space limitations, there are only 64 register frames on the
processor chip.  The high two bits of the Open Frame, Active Frame, and
Return Frame registers are ignored.  There are still 16 global frames,
leaving 48 frames for locals.