Tandy 1000 Technical Reference Manual

Tandy 1000
inter
8259A18259A-218259A-8
Technical Reference Manual
INITIALIZATION COMMAND WORDS 1 AND 2
(ICW1.ICW21
A
s
-A
1S
:
Page starting address of service routines. In an
MCS BO/85 system, the 8 request levels will generate
CALLs to 8 locations equally spaced in memory. These
can be programmed to be spaced at intervals of 4 or 8
memory locations, thus the 8 routines will occupy a
page
of 32 or 64 bytes, respectively.
The address format is 2 bytes long (A
o
-A
1
sl. When the
routine interval is 4, A
o
-A
4
are automatically inserted by
the 8259A, while As-A
1S
are programmed externally.
When the routine interval is 8, Ao-A
s
are
automatically
inserted by the 8259A, while A
6
-A,s are programmed
externally.
The 8·byte interval will maintain compatibility with cur·
rent software, while the 4·byte interval is best for a ::om·
pact jump table.
In an iAPX 86 system
A,s-A" are inserted in the five most
significant bits of the vectoring byte and the 8259A sets
the three least significant bits according to the interrupt
level. A1Q-AS are ignored and ADI (Address interval) has
no effect.
LTlM:
"LTIM=1, then the 8259A will operate in the
level Interrupt mode. Edge detect logic on the
interrupt inputs will be disabled.
ADI: CALL address interval. ADI = 1 then Interval = 4;
ADI = 0 then interval = 8.
SNGL: Single. Means that this is the only 8259A in the
system. " SNGL = 1 no ICW3 will be issued.
IC4: " this bit is set - ICW4 has to be read. " ICW4
Is not needed, set IC4 = O.
INITIALIZATION COMMAND WORD 3 (ICW3)
This word is read only when there is more than one
8259A in the system and cascading is used, in which
case SNGL = O. It will load the 8·bit slave register. The
functions of this register are:
a. In the master mode (either when SP = 1, or in buffered
mode when M/S =1 in ICW4) a "1" is set for each
slave in the system. The master then will release byte
1 of the call sequence (for MCS·80/85 system) and
will enable the corresponding slave to release bytes 2
and 3
(for iAPX 86 only byte 2) through the cascade
lines.
b. In
the slave mode (either when ~ = 0, or If BUF = 1
and
M/S =0 in ICW4) bits 2-0 identify the slave. The
slave
compares its cascade input with these bits and,
if they are equal, bytes 2 and 3 of the call sequence (or
just byte 2 for iAPX 86 are released by it on the Data
Bus.
INITIALIZATION COMMAND WORD 4 (ICW4)
SFNM: " SFNM =1 the special fUlly nested mode is
programmed.
BUF: "BUF = 1 the buffered mode is programmed. In
buffered mode SP/EN becomes an enable outpul
and the master/slave determination is by M/S.
MIS:
"buffered mode is selected: MIS = 1 means the
8259A Is programmed to be a master, MIS = 0
means the 8259A is programmed to be a slave. "
BUF
=0, MIS has no function.
AEOI: "AEOI = 1 the automatic end of interrupt mode
is programmed.
"PM: Microprocessor mode: /LPM = 0 sets the 8259A for
MeS-80, 85 system operation, /LPM = 1 sets the
8259A for iAPX 86 system operation.
,.
CASCADE
MODE
Figure 6. Initialization Sequence
2-126

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