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  1. 1. Microprocessor System SESSION OF MOTOROLA 6800 By Engr.Shafiullah Soomro safiullah123@gmail.com
  2. 2. History of Motorola 6800
  3. 3. ► The 6800 ("sixty-eight-hundred") was an 8-bit  microprocessor designed and first manufactured by  Motorola in 1974. ► The 6800 has a 16-bit address bus that could directly access 64 KB of memory and an 8-bit bi-directional data bus. ► It has 72 instructions with seven addressing modes for a total of 197opcodes. The original MC6800 could have a clock frequency of up to 1 MHz. Later versions had a maximum clock frequency of 2 MHz.
  4. 4. Other Motorola microprocessors
  5. 5. Short Description of Pin Diagram
  6. 6. Pin description
  7. 7. 6800 Architecture
  8. 8. Register Set and Programmers Model ► The 6800 has six internally accessible registers. These are two 8-bit accumulators or general purpose register (A and B), ► three 16-bit registers PC, SP, and ► Index register - X) ► and an 8-bit condition code or status register which has 6 flags in total.
  9. 9. A 8 bit accumulators, general purpose registers B Program Counter - PC Stack Pointer - SP 16 bit registers Index Register - X H I N Z V C 8 bit status register
  10. 10. ► ► ► ► ► ► ► ► The Accumulators A and B : Each stores and manipulates one 8-bit word under program control.   The Index register - X Is a 2-byte register. It holds memory addresses when using indexedaddressing mode instructions.   The Program Counter - PC Is a 2-byte register which contains the address of the next byte of the instruction to be fetched from memory (instructions can be from one to 3 bytes in length). When the current value of the program counter is placed on the address bus, the PC is updated to the value of the next instruction for execution.
  11. 11. ► ► ► ► Stack pointer - SP A 2-byte register which holds the starting address of sequential memory locations in RAM where the contents of the CPU registers may be stored and retrieved. The 6800 uses RAM for its stack. Status Register or Condition Codes Register This final register contains six flags which are set or cleared in response to how the program executes. These flags are:
  12. 12. ► ► ► ► ► ► ► ► ► ► Condition Code Register: It shows the conditions occurs as a result of an Arithmetic Logic Unit Bit 0: carry from bit 7 of an arithmetic operation (C) Bit 1: Overflow flag (V) Bit 2: Zero flag (Z) Bit 3: Negative flag (N) Bit 4: Interrupt Mask (I) Bit 5: Half carry from bit 3 of an arithmetic operation (H) Bit 6: Unused Bit 7: Unused These bits of the Condition Code Register are used as testable conditions for the conditional branch instructions. Bit 4 of the CCR is the interrupt mask bit (I). The unused bits of the Condition Code Register (bit 6 and bit 7) are 1.
  13. 13. ADDRESSING MODES ► ► ► ► ► The 6800 uses a 16-bit address bus allowing access to 65,536 8-bit words within memory. The 16-bit address can be split into a most significant (MS) byte, the first 8-bits of the address and the Least Significant (LS) byte the second 8-bits. For Input Output I/O a portion of memory space is used to access I/O ports.  There are seven addressing modes available to a 6800 programmer. Accumulator Addressing In accumulator addressing, either accumulator A or accumulator B is specified. These are 1-byte instructions. Ex: ABA adds the contents of accumulators and stores the result in accumulator A
  14. 14. ► ► ► ► ► ► Immediate Addressing In immediate addressing, operand is located immediately after the opcode in the second byte of the instruction in program memory (except LDS and LDX where the operand is in the second and third bytes of the instruction). These are 2-byte or 3-byte instructions. Ex: LDAA #25H loads the number (25)H into accumulator A Direct Addressing In direct addressing, the address of the operand is contained in the second byte of the instruction. Direct addressing allows the user to directly address the lowest 256 bytes of the memory, i.e, locations 0 through 255. Enhanced execution times are achieved by storing data in these locations. These are 2-byte instructions. Ex: LDAA 25H loads the contents of the memory address (25)H into accumulator A
  15. 15. ► ► ► Extended Addressing In extended addressing, the address contained in the second byte of the instruction is used as the higher eight bits of the address of the operand. The third byte of the instruction is used as the lower eight bits of the address for the operand. This is an absolute address in the memory. These are 3-byte instructions. Ex: LDAA 1000H loads the contents of the memory address (1000)H into accumulator A
  16. 16. ► ► ► ► ► ► Indexed Addressing In indexed addressing, the address contained in the second byte of the instruction is added to the index register’s lowest eight bits. The carry is then added to the higher order eight bits of the index register. This result is then used to address memory. The modified address is held in a temporary address register so there is no change to the index register. These are 2-byte instructions. Ex: LDX #1000H LDAA 10H,X Initially, LDX #1000H instruction loads 1000H to the index register (X) using immediate addressing. Then LDAA 10H,X instruction, using indexed addressing, loads the contents of memory address (10)H + X=1010H into accumulator A.
  17. 17. ► ► ► ► ► Implied (Inherent) Addressing In the implied addressing mode, the instruction gives the address inherently (i.e, stack pointer, index register, etc.). Inherent instructions are used when no operands need to be fetched. These are 1- byte instructions. Ex: INX increases the contents of the Index register by one. The address information is "inherent" in the instruction itself. INCA increases the contents of the accumulator A by one. DECB decreases the contents of the accumulator B by one.
  18. 18. ► Relative Addressing The relative addressing mode is used with most of the branching instructions on the 6802 microprocessor. The first byte of the instruction is the opcode. The second byte of the instruction is called the offset . The offset is interpreted as a signed 7-bit number . If the MSB (most significant bit) of the offset is 0, the number is positive, which indicates a forward branch. If the MSB of the offset is 1, the number is negative, which indicates a backward branch. This allows the user to address data in a range of -126 to +129 bytes of the present instruction. These are 2-byte instructions. Ex: PC Hex Label Instruction ► 0009 2004 BRA 0FH ► ► ►
  19. 19. Flags