MPU-1
INTRODUCTION
The Motorola M6800 Microcomputer System of standard LSI (Large Scale Integration) devices permits the systems designer to configure and connect a total system with a minimum amount of time and effort. The MC6800 Microprocessing Unit (MPU) forms the nucleus of the system. LSI modules available which may be used to configure a total system in conjunction with the MC6800 MPU, include: 1) MC6810 Random Access Memory (RAM); 2) MC6830 Read Only Memory (ROM); 3) MC6820 Peripheral Interface Adapter (PIA), and 4) MC6850 Asynchronous Communications Interface Adapter (ACIA).
The MPU communicates with the rest of the system via a 16 bit address bus and an 8 bit data bus. The 16 bit address bus provides the MPU the capability of addressing up to 64K. The 8 bit data bus is bi-directional in that data is transferred both into the MPU or out of the MPU over the same bus. A read/write (R/W) line is provided to allow the MPU to control the direction of data transfer.. Since the same bus is used for both data into the MPU or out of the MPU, a separate 8 line bus is saved.
Other features of the M6800 system include a single +5 volt supply, operation at clock rates from 100 kilohertz to 1 megahertz, plus hardware and software interrupt capability.
MPU-2
Microprocessing Unit (MC6800)
The nucleus of the M6800 Microcomputer Family is the microprocessing unit (MPU). The MPU is enclosed in a 40 pin package as shown below:
Features included in the MPU are:
1.Two accumulators (ACCA and ACCB)
2.One index register (X)
3.One program counter register (PC)
4.One stack pointer register (SP)
MPU-3
5.One condition code register (CC)
6.72 instructions
7.Five addressing modes
8.System clock range of 100 kilohertz to 1 megahertz
9.Program interrupt capability
Accumulators
The MPU contains 2 accumulators designated ACCA and ACCB. Each accumulator is 8 bits (one byte) long and is used to hold operands and data from the arithmetic logic unit. Instructions which involve one or both accumulators are:
ABA - Add accumulator A to accumulator B
ADC - Add with carry
ADD - Add without carry
AND - Logical AND
ASL - Arithmetic shift left
ASR - Arithmetic shift right
BIT - Bit test
CBA - Compare accumulators
CLR - Clear
CMP - Compare
COM - Complement
DAA - Decimal adjust ACCA
DEC - Decrement
EOR - Exclusive OR
MPU-4
INC – Increment
LDA - Load accumulator
LSR - Logical shift right
NEA - Negate
ORA - Inclusive OR
PSH - Push data onto stack
PUL - Pull data from stack
ROL - Rotate left
ROR - Rotate right
RTI - Return from interrupt
SBA - Subtract accumulators
SBC - Subtract with carry
STA - Store accumulator
SUB - Subtract
SWI - Software interrupt
TAB - Transfer from accumulator A to accumulator B
TAP - Transfer from accumulator A to processor condition codes register
TBA - Transfer from accumulator B to accumulator A
TPA - Transfer from processor condition codes register to accumulator A
TST - Test
WAI - Wait for interrupt
Index Register
The index register (X) is a 16 bit (2 byte) register which is primarily used to store a memory address in the Indexed mode of memory addressing. The index register may be decremented, incremented and stored. Instructions which involve the index register are:
MPU-5
CPX - Compare index register
DEX - Decrement index register
INX - Increment index register
LDX - Load index register
RTI - Return from interrupt
STX - Store index register
SWI - Software interrupt
TSX - Transfer stack pointer to index register
TXS - Transfer index register to stack pointer
WAI - Wait for interrupt
Program Counter
The program counter (PC) is a 16 bit register that contains the address of the next byte to be fetched from memory. When the current value of the program counter is placed on the address buss, the program counter will be incremented automatically.
Stack Pointer
The Stack Pointer (SP) is a 16 bit (2 byte) register that contains a beginning address, normally in RAM, where the status of the MPU registers may be stored when the MPU has other functions to perform, such as during an interrupt or during a Branch to Subroutine (BTS). The address in the stack pointer is the starting address of sequential memory locations in RAM where MPU status registers will be stored. The status of the MPU will be stored in the RAM as follows:
Stack Pointer Address : contents of PCL
Stack Pointer Address-1 : contents of PCH
Stack Pointer Address-2 : contents of IXL
Stack Pointer Address-3 : contents of IXH
Stack Pointer Address-4 : contents of ACCA
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MPU-6 |
Stack |
Pointer |
Address-5 |
: Contents |
of ACCB |
Stack |
Pointer |
Address-6 |
: Contents |
of CC |
After the status of each register is stored on the stack, the Stack Pointer will be decremented. When the stack is unloaded (status of registers restored), the status of the last register on the stack will be the first register that is restored.
Condition Code Register (CC)
The condition code register is an 8 bit register. Each individual bit may get set or get cleared from execution of an instruction. To see how each instruction effects the condition code register, refer to the M6800 programming manual. The primary use of this register is execution of the conditional branch instruction. Bit 6 and 7 are not used and remain at logic "1."
BIT N0. |
FUNCTION |
0C (Carry-Borrow Test)
1V (Overflow Test)
2Z (Zero Test)
3N (Negative Test)
4I (Interrupt Mask Test)
5H (Half Carry Test)
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MPU-7 |
Carry-Borrow: |
For addition, the carry-borrow condition code (C) in the |
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zero bit position, represents a carry. This bit gets set |
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(C=1) to indicate a carry, and is reset (C=0) if there is |
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no carry. |
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For subtraction, the C bit is set (C=1) to indicate a |
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borrow and is reset (C=0) to indicate there was no borrow. |
Overflow: |
The V bit (bit 1) of the condition code register is set |
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(V=1) when two's complement overflow results from an |
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arithmetic operation, and is reset (V=O) if two's |
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complement overflow does not occur. |
Zero: |
The Z bit (bit 2) of the condition code register is set |
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(Z=1) if the result of an arithmetic operation is zero, and |
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is reset (Z=0) if the result is not zero. |
Negative: |
The N bit (bit 3) of the condition code register is set |
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(N=1) if bit 7 of an arithmetic operation is set (equal to |
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1). This indicates that the two's complement number, |
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represented by the bit pattern of the result, is negative. |
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The N bit is reset (N=0) if bit 7 of the arithmetic result |
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is equal to 0. |
Interrupt Mask: If this I bit (bit 4) is set (I=1), the MPU cannot respond
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to an interrupt request from any peripheral device. |
Half-Carry: |
The half carry bit H (bit 5) of the condition code register |
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is set (H=1) during execution of any of the instructions |
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ABA,ADC, or ADD, if there is a carry from bit position 3 to |
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bit position 4. The half carry is reset (H=0) during these |
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operations, if there is no carry from bit position 4. |
MPU-8
MPU Signal Descriptions
1. READ/WRITE (R/W):
This output line is used to signal all devices external to the MPU that the MPU is in a read state (R/W = High) or a write state (R/W = Low). The normal standby state of this line when no external devices are being accessed is a high state. This line is three-state. When three-state goes high, this line enters the high impedance mode.
2. VALID MEMORY ADDRESS(VMA):
This output line, (when in the high state) tells all devices external to the MPU that there is a valid address in the address bus. For RAM's and ROM's, this line should be ANDed with 12 clock and used as one of the enables. For PIA's, this line should be ANDed with one of the PIA address lines. This signal is not three-state.
3. DATA BUS ENABLE(DBE):
This signal will enable the data bus drives when in the high state. This input is normally the phase 2 (12) clock. During the high state, it will permit data to be output during a write cycle. During an MPU read cycle, the data bus drives will be disabled internally.
4. INTERRUPT REQUEST(IRQ):
This input from the PIA's requests that an interrupt sequence be generated within the machine. The processor will wait until it completes the current instruction that is being executed before it recognizes the request. At that time, if the interrupt mask bit in the Condition Code Register is not set (interrupt masked), the machine will begin an interrupt sequence. The Index Register, Program
MPU-9
Counter, Accumulators, and Condition Code Register are stored away on the stack. Next the MPU will respond to the interrupt request by setting the interrupt mask bit high so that no further interrupts may occur. At the end of the cycle, a 16-bit address will be loaded that points to a vectoring address which is located in memory locations n-6 and n-7 where n is the highest ROM address. An address loaded at these locations causes the MPU to branch to an interrupt routine in memory.
5. Phase One (Øl)& Phase (Ø2)Clocks:
These two pins are used or a two phase non-overlapping clock that runs at the V DD voltage level.
These clocks run at a rate up to 1 megahertz.
6. Restart (RES):
RESTART (RES)--This input is used to start the MPU from a power down condition, resulting from a power failure or an initial start-up of the processor. If a positive edge is detected on the input, this will signal the MPU to begin the restart sequence. This will restart the MPU and start execution of a routine to initialize the processor. All the higher order address lines will be forced high. For the restart, the last 2 memory locations in the last ROM (n&n-1) will be accessed, whereby an address is stored which is the address to be loaded in the program counter which tells the processor where program execution is to begin.
7. NON-MASKABLE INTERRUPT(NMI):
This input requests that a nonmask-interrupt sequence be generated within the processor. As with the Interrupt
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