Page 197 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 14: Interrupts

• CALL instruction (3 byte instruction). The processor calls the subroutine, address of which is Notes
specified in the second and third bytes of the instruction.
RST5.5 is a maskable interrupt. When this interrupt is received the processor saves the contents
of the PC register into stack and branches to 2Ch (hexadecimal) address.
RST6.5 is a maskable interrupt. When this interrupt is received the processor saves the contents
of the PC register into stack and branches to 34h (hexadecimal) address.
RST7.5 is a maskable interrupt. When this interrupt is received the processor saves the contents
of the PC register into stack and branches to 3Ch (hexadecimal) address.
Trap is a non-maskable interrupt. When this interrupt is received the processor saves the contents
of the PC register into stack and branches to 24h (hexadecimal) address.

All maskable interrupts can be enabled or disabled using EI and DI instructions. RST 5.5, RST6.5
and RST7.5 interrupts can be enabled or disabled individually using SIM instruction.

I/O ports
256 Input ports
256 Output ports
Registers

Accumulator or A register is an 8-bit register used for arithmetic, logic, I/O and load/store
operations.

Flag is an 8-bit register containing 5 1-bit flags:
• Sign - set if the most significant bit of the result is set.
• Zero - set if the result is zero.

• Auxiliary carry - set if there was a carry out from bit 3 to bit 4 of the result.
• Parity - set if the parity (the number of set bits in the result) is even.
• Carry - set if there was a carry during addition, or borrow during subtraction/comparison.
General registers:

• 8-bit B and 8-bit C registers can be used as one 16-bit BC register pair. When used as a pair the
C register contains low-order byte. Some instructions may use BC register as a data pointer.
• 8-bit D and 8-bit E registers can be used as one 16-bit DE register pair. When used as a pair the
E register contains low-order byte. Some instructions may use DE register as a data pointer.
• 8-bit H and 8-bit L registers can be used as one 16-bit HL register pair. When used as a pair
the L register contains low-order byte. HL register usually contains a data pointer used to
reference memory addresses.
Stack pointer is a 16 bit register. This register is always incremented/decremented by 2.
Program counter is a 16-bit register.
Instruction Set

Instruction set of Intel 8085 microprocessor consists of the following instructions:
• Data moving instructions.
• Arithmetic - add, subtract, increment and decrement.

• Logic - AND, OR, XOR and rotate.

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