Tandy 1000 Technical Reference Manual

Tandy 1000
Technical Reference Manual
intel
8259A/8259A-2/8259A-8
B.lor. Rotlt. (IA4 the highest priority requrrmg service)
AUTOMATIC ROTATION
(Equal Priority Devices)
In some applications there are a number 01 Interrupting
devices 01 equal priorrty. In this mode a device, after
being serviced, rece,ves the lowest priorrty, so a device
requestmg an Interrupt Will have to walt, In the worst
case until each 01 7 other devices are serviced at most
once For example, If the priority and "In service" status
IS
FULLY NESTED MODE
This mode is entered after initialization unless another
mode is programmed. The interrupt requests are
ordered in
priority form 0 through 7 (0 highest) When an
interrupt is acknowledged the highest priorrty request is
determined and its vector placed on the bus. Additional·
Iy, a bit of the Interrupt Service register (ISO-7) is set
This
bit remains set until the microprocessor issues an
End
of Interrupt (EOI) command immediately before
returning from the service routme, or if AEOI (Automatic
End of Interrupt) bit IS set, until the trailing edge of the
last INTA. While the IS bit is set, all further interrupts of
the same or lower priority are inhibited, while higher
levels will generate an interrupt (which will be
acknowledged only" the microprocessor internal Inter-
rupt
enable flip-flop has been re-enabled through soft-
ware)
After the Initialization sequence, lAO has the hlgnest
priority and IA7 the lowest Priorities can be changed, as
will be explained, in the rotating priority mode
IS'· SlaluS
Priority Sla1U5
'S7 lSI IS5 I~ IS3 IS2 lSI ISO
COE'E.ElOTiIYT~J
Low •• ' Pnority HIQh." Priority
LihL~l_~Elir~
High •• , Prtortty Low.,' Prlorily
Lz_l~~f!J1l~_IiJ
'IS" Sialus
PriorIty SlaluS
Aft.r Rotlt. (IA4 was serviced, all other prrorrtles
rotated
correspondingly)
IS7 lSI 'S5 ,~ IS3 IS2 lSI ISO
[01.-'.T~[aI~D.I~
INTERRUPT MASKS
Each Interrupt Aequest input can be masked individu-
ally by the Interrupt Mask Aegister (IMA) programmed
through OeW1. Each bit in the IMA masks one interrupt
channel if it is set (1). Bit 0 masks lAO. Bit 1 masks IA1
and
so forth. Masking an IA channel does not affect the
other channels operation.
There are two ways to accomplish Automatic Rotation
using OCW2, the Rotation on Non-SpecifiC EOI Command
(R ~ 1, SL. 0, EOI - 1) and the Rotate in Automatic EOI
Mode which is set by (R = 1. SL ~ 0, EOI = 0) and cleared
by
(R = 0, SL ~ 0, EOI = 0)
SPECIFIC ROTATION
(Specific Priority)
The programmer can change prrorrtles by programmmg
the bottom priorrty and thus fixmg all other prrorrtles;
i.e, If IA5 is programmed as the bottom prrorrty deVice,
then IA6
will have the highest one.
The Set Pflority command is issued In OCW2 where:
R ~ 1, SL = 1: LO-L2 is the binary priority level code olthe
bottom priorrty deVice
Observe that In this mode internal status IS updated by
software control during OCW2. However, it is independent
of the End of Interrupt (EOI) command (also executed by
OCW2).
Priority changes can be executed durrng an EOI
command by uSing the Rotate on Specific EOI command
in OCW2 (R = 1, SL ~ 1, EOI ~ 1 and LO-L2 = IR level to
receive bottom priority)
END OF INTERRUPT (EOI)
The
In Service (IS) bit can be reset either automatically
follOWing the trailing edge of the last in sequence INTA
pulse (when AEOI bit In ICW1 is set) or by a command
word that must be issued to the 8259A before returning
from a service routine (EOI command). An EOI command
must be Issued tWice if In the Cascade mode, once for the
master and once for the corresponding slave
There are two forms of EOI command: Specific and Non-
Specific When the 8259A IS operated In modes which
preserve the fUlly nested structure, it can determine
which IS bit to reset on EOI When a Non-Specific EOI
command IS issued the 8259A will automatically reset
the highest IS bit of those that are set, since In the
fUlly nested mode the highest IS level was necessarily the
last level
acknowledged and serviced. A non-specific EOI
can be issued With OCW2 (EOI ~ 1, SL = 0, R ~ 0)
When a mode IS used which may disturb the fully nested
structure,
the 8259A may no longer be able to determine
the last level acknowledged. In thiS case a Specific End of
Interrupt must be issued which Includes as part of the
command the IS level to be reset A specific EOI can be IS-
sued with OCW2 (EOI ~ 1, SL = 1, R = O. and LO-L2 is the
binary level of the IS bit to be reset)
It should be noted that an IS bit that IS masked by an
IMA
bit will not be cleared by a non-speCifiC EOI II the
8259A is In the Special Mask Mode
AUTOMATIC END OF INTERRUPT (AEOI) MODE
If AEOI = 1 In ICW4, then the 8259A Will operate in AEOI
mode
continuously until reprogrammed by ICW4. In this
mode the 8259A will automatically perform a non-
specific EOI operation at the trailing edge of the last
interrupt acknowledge pulse (third pulse In MCS-80i85,
second in iAPX 86) Note that from a system standpoint,
this mode should be used only when a nested multilevel
interrupt structure is not required within a single 8259A.
The AEOI
mode can only be used in a master 8259A and
not a slave
2-130

Tandy 1000 Specifications

General

Processor	Intel 8088
Processor Speed	4.77 MHz
RAM	128 KB (expandable to 640 KB)
Operating System	MS-DOS 2.11
Manufacturer	Tandy Corporation
Release Year	1984
Storage	Single or double 5.25" floppy disk drive (360 KB), optional hard drive
Sound	Tandy 3-voice sound
Ports	Parallel, Serial
Graphics	CGA (Color Graphics Adapter)
Graphics Modes	320x200, 640x200