Command words of 8259 PIC

Command word of 8259 is divided into two parts :

• Initialization command words(ICW)
• Operating command words(OCW)

Initialization command words(ICW) :

• ICW is given during the initialization of 8259 i.e. before its start functioning.
• ICW₁ and ICW₂ commands are compulsory for initialization.
• ICW₃ command is given during a cascaded configuration.
• If ICW₄ is needed, then it is specified in ICW₁.
• The sequence order of giving ICW commands is fixed i.e. ICW₁ is given first and then ICW₂ and then ICW₃.
• Any of the ICW commands can not be repeated, but the entire initialization process can be repeated if required.

Operating command words(OCW) :

• OCW is given during the operation of 8259 i.e. microprocessor starts using 8259.
• OCW commands are not compulsory for 8259.
• The sequence order of giving OCW commands is not fixed.
• The OCW commands can be repeated.

Initialization sequence of 8259 :

ICW₁ command :

• The control word is recognized as ICW₁ when A₀ = 0 and D₄ = 1.
• It has the control bits for Edge and level triggering mode, single/cascaded mode, call address interval and whether ICW4 is required or not.
• Address lines A₇ to A₅ are used for interrupt vector addresses.

When the ICW₁ is loaded, then the initializations performed are:

• The edge sense circuit is reset because, by default, 8259 interrupt is edge triggered.
• The interrupt mask register is cleared.
• IR7 is assigned to priority 7.
• Slave mode address is assigned as 7.
• When D₀ = 0, this means IC₄ command is not required. Therefore, functions used in IC4 are reset.
• Special mask mode is reset and status read is assigned to IRR.

ICW₂ command :

• The control word is recognized as ICW₂ when A₀= 1.
• It stores the information regarding the interrupt vector address.
• In the 8085 based system, the A15 to A₈ bits of control word is used for interrupt vector addresses.
• In the  8086 based system, T₆ to T₃ bits are inserted instead of A₁₅ to A₈ and A₁₀ to A₈ are used for selecting interrupt level, i.e. 000 for  IR₀ and 111 for IR₇.

Initialization of 8259 by ICW1 and ICW2 command words

ICW₃ :
ICW₃ command word is used when there is more than one 8259 present in the system i.e. when SNGL bit in ICW₁ is 0, then it will load 8-bit slave register.

ICW3

ICW₄ :

• When AEOI = 1, then Automatic end of interrupt mode is selected.
• When SFMN = 1, then a special fully nested mode is selected.
• when BUF = 0 , then Non buffered mode is used (i.e. M/S is don’t care) and when M/S = 1, then 8259 is master, otherwise it is a slave.
• when µPM = 1, then 8086 operations are performed, otherwise 8085 operations are performed.

ICW4

Operational command word(OCW) :

OCW_{1 –}
It is used to set and reset the mask bits in IMR(interrupt mask register). M₇ – M₀ describes 8 mask bits

OCW₂ –
It is used for selecting the mode of operation of 8259. Here L₂ to L₀ are used to describe interrupt level on which action need to be performed.

Detailed operations are described in the diagram below. 

OCW₃ –

• When the ESMM (Enable special mask mode ) bit is set, then the SMM bit is don’t care. If SMM = 1 and ESMM = 1, then 8259 will enter in Special mask mode.
• If ESMM = 1 and SMM = 0, then 8259 will return into normal mask mode.
• RR and RIS are used to give the read register command.
• P = 1 is used for poll command.