The Microprocessors The Rockwell 6502 is an 8 bit microprocessor and has a bus system directly compatible with the Motorola 6800 series of integrated circuits.
Rockwell 6502 • Fig 5.1 shows a functional block diagram of Rockwell 6502 microprocessor, the registers and the pin connections. • The 6502 has five internal registers: • An 8 bit accumulator • A status (flag) register • Two 8 bit X and Y registers • A stack pointer • A program counter
Rockwell 6502 .. • The 6502 is packaged in a standard 40-pin package. • Address Bus – A0 to A15 constitute a 16 bit address bus • Data Bus – D0 to D7 constitute an 8 bit bidirectional data bus • System Control – The R/W pin is taken high, i.e. 1 to read, 0 to write
Rockwell 6502 .. • Processor Control • Three Interrupt lines: • RES line to reset the microprocessor • IRQ is used for interrupt request • NMI (non maskable interrupt) can not be turned off • Clock - Φ0 is for single phase external crystal-controlled oscillator. The Φ1 and Φ2 are used to synchronize operations
Motorola 6800 • First 8 bit microprocessor from Motorola and needed an external clock generator • The later 6802 incorporates a built-in clock generator • Two accumulator register, a status register, an index register, a stack pointer register and a program counter
Motorola 6800 • The status register has flag bits to show negative N, zero Z, carry C, overflow V, half-carry H, and Interrupt I. The half-carry flag is used in binary coded decimal arithmetic and is set to 1 if a carry is generated between bit 3 and bit 4 of a calculation • Address Bus – A0 to A15 (Tri-state Outputs) • Data Bus – D0 to D7 (Tri-state Outputs)
Motorola 6800 • Control • Pins 40, 6, 2, 4, 39, 36, 7, 5, and 34 are used for control • The valid memory address output on pin 5 signals to memory that there is a valid address on the address bus • The data bus enable input on pin 36 is used to enable other devices to take control of the data bus • A bus available output on pin 7 indicates that the address bus is available