The Microprocessors

1
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.

2
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

3

• Fig. 5-1

4

• Fig. 5-1 ..

5
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

6
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 - F0 is for single phase external
  crystal-controlled oscillator. The F1 and F2 are
  used to synchronize operations

7
Motorola 6800
8
Motorola 6800
9
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

10
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)

11
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