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Alto Hardware Manual
4
5
6
7
Task-specific
"'MD
... MOUSE
"'DISP
Section 2: Microprocessor
Performs different functions in different tasks.
Memory data
Bus[12-15]+- MOUSE; BUS[O-13]+- -1
IR[8-15], possibly sign extended (see section 3.5)
5
RName+- is not logically a source, but because it is gated to the bus during both reading "and writing, it is
included in the source specifiers. Loading R forces the BUS to
0
so that an ALU function of 0 and T may
be executed simultaneously.
The bus has the property that if more than one source is gated to it during a single microinstruction, it
computes the AND of the source values. This is true regardless of the means by which the sources are
enabled (BS, Fl, or F2).
This bus source decoding is not performed if Fl=7 or F2=7. These functions use the BS field to provide
part of the address to the constant ROM.
SPECIAL FUNCTIONS
The two function fields specify the address modifiers, register load signals (other than those for R, S, L, M
and T), and other special conditions required in the processor. The first eight conditions specified by
each field (except BLOCK) are interpreted identically by all tasks, but the interpretation of the second
eight depends on the active task.
The task-independent functions are given below; the task-specific
functions are included with the task descriptions.
FUNCTION 1:
Fl
o
1
2
3
4
5
6
7
NAME
MAR'"
TASK
BLOCK
"'L LSH
1
"'L RSH
1
"'L LCY8
"'CONSTANT
MEANING
No Activity
Load MAR from ALU output; start main memory reference (see section
2.3).
Switch tasks if higher priority wakeup is pending (see section 2.4).
Disable the current task until re-enabled by a hardware-generated
condition. Note: this function is reserved by convention only; it is not
done by the microprocessor.
SHIFTER OUTPUT will be L shifted left one place*
SHIFTER OUTPUT will be L shifted right one place*
SHIFTER OUTPUT will be L rotated left
8
places*
Put on the bus the constant from the constant ROM location addressed by
RSELECT.BS
*Modified by DNS (DO Novel Shifts) function, and MAGIC function.
L LSH land L RSH
1
ordinarily shift a zero into the vacated bit position.
FUNCTION 2:
F2
NAME
MEANING
0
No Activity
1
BUS=O
NEXT'" NEXT OR (if (BUS=O) then 1 else 0).
2
SH(O
NEXT'" NEXT OR (if (SHIFTER OUTPUT(O) then 1 else 0).*
3
SH=O
NEXT'" NEXT OR (if (SHIFTER OutPUT=O) then 1 else 0).*
4
BUS
NEXT'" NEXT OR Bus[6-15]
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