Xerox Alto I Hardware Manual page 67

Alto Hardware Manual
NAME
MI RaMO·
MI RaMI
MI RAMO
MI RAMI
MI RAM2
Section 8: Control
RAM,
ROM, and
s
Registers
WHAT
The standard microcode ROM.
Second bank of ROM in the 2K ROM configuration.
The standard microcode writeable RAM.
Second bank of RAM in the 3K RAM configuration.
Third bank of RAM in the 3K RAM configuration.
58
Switching among banks is controlled in two ways: (1) a RAM related task already running may "switch"
banks, and (2) it possible to initiate a task in either ROMO or RAMO.
Bank switching is accomplished with a special transfer mechanism, available only to the emulator task, in
the form of SWMODE, a RAM-related
Fl.
SWMODE will'switchthe bank cjf the running task, taking effect
after the microinstruction following that in which the SWMODE appears. In other words, the emulator
task SWMODE behaves much like an address modifier. Tasks other than the emulator cannot switch
banks. The effect of SWMODE depends on the ROMIRAM configuration, the bank in which the task is
currently executing, and the value of NEXT in the instruction following the one that asserts SWMODE.
In the 1K RAM configuration (neither the 2K ROM nor the 3K RAM option installed):
If currently go to NEXT in'
executing in
RaMO
RAMO
RAMO
ROMO
In the 2K ROM configuration (which includes 1K of RAM):
If currently
and NEXT[l J = 0 then
else
executing in go to NEXT in go to NEXT in
RaMO RAMO ROMI
RaMI . RaMO RAMO
RAMO RaMO ROMI
In the 3K RAM configuration:
If currently
executing in
NEXT[lJ=O NEXT[lJ=l
ROMO
RAMO
RAMI
RAM2
NEXT[2J=0 NEXT[2J=1
RAMO
ROMO
ROMO
ROMO
RAM2
RAM2
RAM2
RAMI
NEXT[2] =0 NEXT[2J=1
RAMI
RAMI
RAMO
RAMO·
RAMO
RAMI
RAMO
RAMO
If the table above determines that control is to be transferred to the RAM, and the RAM is not installed.
control remains in the bank in which the task is currently executing.
Many Alto IIs have the 2K ROM capability but contain nothing in ROMI. In these Altos, the SWMODE
operation is normally configured so that it behaves as if RaMI didn't exist (Le., according to the first
table rather than the second). This is determined by the chip in position 51 on the control board. If it is
labelled SW2K then RaMI exists, but if SWIK then it does not. The alternate chip is kept in unused
socket 76.
SWMODE is actually defined in all RAM-related tasks, not just the' emulator; however, it does not work
correctly in tasks other than the emulator in Altos with the 2K ROM or 3K RAM configuration.
Each of the 16 micro-tasks may be started either in RaMO or in RAMO when a hardware reset
("bootstrap") operation is performed, regardless of whether the task is RAM-related A 16-bit "reset mode