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MEMORY
Disk
VO
program
SIO
I
lOP
Interrupt
routine
VO
Control Doubleword
Machine fault
routine
Read Direct
t
I
i
j'
I , •
,I. , ~~:il
"
J
.I
Second data
I
First da!'o by tel
byte
0 1 2 1 . 5 0
i
I
It IJIJ'.'5
Data byte
llast data byte
I
~
I
2
,,!i,
I ,
I
11,12 13 I"
:$
VO
Channel Register Pair
even
odd
I
I
Device No.
I
6
i
J Sl.
i . ,
I. , ,
,I. , . ,:.
i
i .
Hid
'J . . .
J.' i
)"20
,,~r.~:~;!Il"'!Ii,J
CONTROLLER
I
L
-
I~\:H~ ~l+lo
I
0 \ S il. 3.
,I
[±]
.
J '
.
Order In Status
Device Status Byte
Overflow, Carry
Address Register
I
T rack Sector
I
I
I
ORNE
I
I
I
~
~
~
~
Disk
CPU
A Register
~
Overflow,
Carry
I.
I ,
,I.
! •
i
I ..
I. ill
Ii
IS I •
.J
Each disk has
a
.unique device address. CPU executes
instructions; lOP executes commands; ond
the
controller
and device execute orders.
Orders
X'Ol'
Write
X'02'
Read
X'03'
Seek
X'04'
Sense
X'OS'
Check-write
X'12'
Read
X'13'
Test
I/O Control Doubleword
Word address
E
DC
I
Byte count
Pointer to the first word of the
VO
tabl.
Error flag
Data chaining flag
Interrupt flag
Number of bytes to be tran$lftitted.
Inel udes order byte.
Device No. Controller
a~d
device addresses.
Order In Status. Set at the conclusion of an
VO
opera-
tion such as a read/write order. The flags
ant:
TE
IL
CE
UE
Transmission error
Incorrect length
Channel end
Unusual end
DeVice Status Byte. Set as the result of an I/O instruc-
tion.
Reports condition of device and controller, such
as not operational, ready, busy.
Overflow and Carry Indicators. Set as the result of an
VO
instruction.
Reports, e.g., address recognizttcl,
VO
instruction accepted or nat accepted.
Address Register. Track and sector addresses set initially
by a Seek order.
Addresses automatically incremented
by the controller.
Figure 5. I/O Operational Flow - Xerox 530 and Sigma 3
I/O Operational Flow
7
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