Digital Electronics: Principles, Devices and Applications

542 Digital ElectronicsX1X2RESET OUTSODSIDTRAPRST 7.5RST 6.5RST 5.5INTRINTAAD0AD1AD2AD3AD 4AD 5AD 6AD7GND12345678910111213141516171819201 24 40 Vcc39 HOLD383736353433323130292827262524232221HLDACLK(OUT)RESET INREADY10/MS1RDWRALES0A15A14A13A12A11A10A9A8Figure 13.11 Pin-out configuration of 8085.13.9.1.1 8085 RegistersThe 8085 microprocessor registers include an eight-bit accumulator, an eight-bit flag register (fiveone-bit flags, namely sign, zero, auxiliary carry, parity and carry), eight-bit B and C registers (whichcan be used as one 16-bit BC register pair), eight-bit D and E registers (which can be used as one16-bit DE register pair), eight-bit H and L registers (which can be used as one 16-bit HL register pair),a 16-bit stack pointer and a 16-bit program counter.13.9.1.2 Addressing Modes8085 has four addressing modes. These include register addressing, register indirect addressing, directaddressing mode and immediate addressing mode.13.9.1.3 8085 InstructionsAn instruction is a binary pattern designed inside a microprocessor to perform a specific function.The entire group of instructions a microprocessor can perform is referred to as its instruction set. Aninstruction cycle is defined as the time required to complete the execution of an instruction. An 8085instruction cycle consists of 1–6 machine cycles. A machine cycle is defined as the time required tocomplete one operation of accessing memory, I/O and so on. This will comprise 3–6 T-states, whichis defined as one subdivision of the operation performed in one clock period.