Title: String instructions
1String instructions
2MOVS (MOVe String)
3MOVS (MOVe String) Contd.
4MOVS (MOVe String) Contd.
5Use of REP prefix and MOVSB
6CMPS (CoMPare Strings)
7CMPS Contd.
8CMPS Contd.
9CMPS Contd.
10CMPS Contd.
11CMPS Contd.
12STOS (STOre String) instruction
13STOS Contd.
14STOS Contd.
15STOS Contd.
16LODS (LOaD String) instruction
17LODS Contd.
18LODS Contd.
19LODS Contd.
20SCAS (SCAn String) instruction
21SCAS Contd.
22SCAS Contd.
23SCAS Contd.
24SCAS Contd.
25SCAS Contd.
26Intel 8255 PPI
27Intel 8255 PPI
Chip Select Circuit
A7
M/IO
288255 PPI contd.
298255 PPI Contd.
308255 PPI Contd.
318255 PPI Contd.
328255 PPI Contd.
338255 PPI Contd.
348255 Handshake signals
358255 Handshake signals Contd.
368255 Contd.
378255 MD Control word
388255 MD Control word Contd.
398255 MD Control word Contd.
408255 MD Control word Contd.
418255 PCBSR Control word
428255 PCBSR Control word contd.
438255 PCBSR Control word contd.
448255 PCBSR Control word contd.
458255 Mode1 operation
46Handshake Interrupt i/p port
For Port A as handshake interrupt input
port INTA is PC3 STBA is PC4 IBFA is PC5
STBB (PC2)
47Handshake Interrupt i/p port
48Handshake interrupt i/p port
When i/p device has data to send it checks if IBF
(input buffer full) signal is 0. If 0, it sends
data on PB7-0 and activates STB (Strobe) signal.
STB is active low. When STB goes high, the
data enters the port and IBF gets activated. If
the Port interrupt is enabled, INT is activated.
This interrupts the processor. Processor reads
the port during the ISS. Then IBF and INT get
deactivated.
49Handshake interrupt o/p port
8255
For Port A as handshake interrupt output
port INTA is PC3 ACKA is PC4 OBFA is PC5
ACKB (PC2)
50Handshake interrupt o/p port
51Handshake interrupt o/p port
When o/p device wants to receive data it checks
if OBF (output buffer full) signal is 0. If 0,
it receives data on PB7-0 and activates ACK
(Acknowledge) signal. ACK is active low. When
ACK goes high, the data goes out of the port and
OBF is set to 1. If the Port interrupt is
enabled, INT is activated. This interrupts the
processor. Processor sends another byte to the
port during the ISS. Then OBF and INT are reset
to 0.
52Handshake Status Check I/O
Interrupt is disabled for the port using
PCBSR Even if new data is entered into I/p buffer
by I/O device INT o/p is not going to be
activated for i/p operation How processor knows
that the i/p buffer has new data? Even if I/O
device has emptied the o/p buffer, INT o/p is not
going to be activated for o/p operation How the
processor knows that the o/p buffer is
empty? Processor reads the status of the port
for this purpose
53Port C as provider of Status
54Handshake status check i/p port