Xerox Alto I User Manual

Alto Hardware Manual Section 3: Emulator
15
The alert reader will detect that the SK field is microcoded. The skip condition can be described as:
skip =
(SK[2):;e0) XOR
«SK[O):;eO AND result = 0) OR (sK[l):;eO AND carryResult= 0))
where SK[O) is the first bit of the field, SK[1] the second and SK[2]the third.
The NL bit in the instruction controls the operation of the switch in the illustration. If NL= 1, neither the
destination accumulator nor the carry bit is loaded; otherwise the destination accumulator is loaded from
Result
and the carry bit from carryResult The "no-load" feature is useful for instructions whose only
use is testing some value.
The character # is appended to the mnemonic for operations if the NL bit is
to
be set
The AFUnc operations are described below. Note that "Result" will be stored into the destination
accumulator
(DeStAc) unless NL=l.
Mnemonic AFUnc Operation
Description
COM
NEG
MOV
INC
ADC
SUB
ADD
AND
o
1
2
3
4
5
6
7
COMPLEMENT The function generator produces the logical complement
of Ac(srcAc). It passes the carry bit unaffected.
NEGATE The function generator produces the two's complement
of Ac(srcAc). If Ac(srcAc) contains zero, complement
the value
of the carry supplied to the function generator,
otherwise supply the specified value.
MOVE The function generator passes Ac(srcAc) and the carry
bit unaffected.
INCREMENT The Result produced ·.is AC(SrcAC) + 1; the carry is
complemented
if Ac(srcAc) = 177777B.
ADD COMPLEMENT The Result produced is the sum of AC(DeStAc) and
the logical complement of AC(SfCAC). The carry bit is
complemented
if the addition generates a carry.
SUBTRACT Subtracts by adding the two's complement of Ac(srcAc)
to
AC(DeStAc). The carry bit is complemented if the
addition generates a carry ..
ADD Adds Ac(srcAC) to AC(DestAc). The carry bit is
complemented
if the addition generates a carry.
AND The Result is the logical and of Ac(srcAC) and
AC(DeStAc). The carry is passed unaffected.
The arithmetic instructions are written by citing. the AFUnc mnemonic, followed optionally by the CY
mnemonic, followed optionally by the SH mnemonic, followed optionally by the NL mnemonic. Then
after a space, the source accumulator number is given, the destination accumulator number, and
optionally an SK mnemonic. For example:
SUB 0 0
MOVZ 2 1
SUBZL 1 1
ADC 0 0
SUB# 2 3 SNR
COM# 1 1 SZR
SUBZ# 1 0 SZC
ADCZ# 1 0 SZC
Zero ACO by subtracting it from itself
Move AC2 to AC1, and zero C
Set AC1 to 1
Set ACO to 177777B
Skips if AC2 and AC3 are unequal but
affects neither
Skips if AC1 is 177777B but leaves it unchanged
Skips if ACO(AC1 unsigned.
Skips if ACOiAC1 unsigned
To subtract the constant 1 from Ac1:

Xerox Alto I Specifications

General

Brand	Xerox
Model	Alto I
Category	Desktop
Language	English