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The MC68000 was a complete design from scratch with the emphasis on providing an architecture that looked forward with-out the restrictions of remaining compatible with past designs. Unlike the Intel approach of taking an 8 bit architecture and developing it further and further, Motorola's approach was to design a 16/32 bit processor whose architecture was more for-ward looking.
The only support for the old MC6800 family was a hard-ware interface to allow the new processor to use the existing M6800 peripherals while new M68000 parts were being designed.
Its design took many of the then current mini and main-frame computer architectural concepts and developed them using VLSI silicon technology. The programmer’s register model shows how dramatic the change was. Gone are the dedicated 8 and 16 bit registers to be replaced by two groups of eight data registers and eight address registers. All these registers and the program coun-ter are 32 bits wide.
The MC68000 hardware
The address bus (signals A1 – A23) is non-multiplexed and 24 bits wide, giving a single linear addressing space of 16 Mbytes. A0 is not brought out directly but is internally decoded to generate upper and lower data strobes. This allows the processor to access either or both the upper and lower bytes that comprise the 16 bit data bus.
The data bus, D0 – D15, is also non-multiplexed and pro-vides a 16 bit wide data path to external memory and peripherals. The processor can use data in either byte, word (16 bit) or long word (32 bit) values. Both word and long word data is stored on the appropriate boundary, while bytes can be stored anywhere. The diagram shows how these data quantities are stored. All addresses specify the byte at the start of the quantity.
If an instruction needs 32 bits of data to be accessed in external memory, this is performed as two successive 16 bit accesses automatically. Instructions and operands are always 16 bits in size and accessed on word boundaries. Attempts to access instructions, operands, words or long words on odd byte bounda-ries cause an internal ‘address’ error.
The function codes, FC0–FC2, provide extra information describing what type of bus cycle is occurring. These codes and their meanings are shown in the table below. They appear at the same time as the address bus data and indicate program/data and supervisor/user accesses. In addition, when all three signals are asserted, the present cycle is an interrupt acknowledgement, where an interrupt vector is passed to the processor. Many design-ers use these codes to provide hardware partitioning.
Seven interrupt levels are supported and are encoded on to three interrupt pins IP0–IP2. With all three signals high, no exter-nal interrupt is requested. With all three asserted, a non-maskable level 7 interrupt is generated. Levels 1–6, generated by other combinations, can be internally masked by writing to the appro-priate bits within the status register.
The interrupt cycle is started by a peripheral generating an interrupt. This is usually encoded using a 148 priority encoder. The appropriate code sequence is generated and drives the inter-rupt pins. The processor samples the levels and requires the levels to remain constant to be recognised. It is recommended that the interrupt level remains asserted until its interrupt acknowledge-ment cycle commences to ensure recognition. Once the processor has recognised the interrupt, it waits until the current instruction has been completed and starts an interrupt acknowledgement cycle. This starts an external bus cycle with all three function codes driven high to indicate an interrupt acknowledgement cycle.
The interrupt level being acknowledged is placed on ad-dress bus bits A1–A3 to allow external circuitry to identify which level is being acknowledged. This is essential when one or more interrupt requests are pending. The system now has a choice over which way it will respond:
• If the peripheral can generate an 8 bit vector number, this is placed on the lower byte of the address bus and DTACK* asserted. The vector number is read and the cycle com-pleted. This vector number then selects the address and subsequent software handler from the vector table.
• If the peripheral cannot generate a vector, it can assert VPA* and the processor will terminate the cycle using the M6800 interface. It will select the specific interrupt vector allocated to the specific interrupt level. This method is called auto-vectoring.
To prevent an interrupt request generating multiple ac-knowledgements, the internal interrupt mask is raised to the interrupt level, effectively masking any further requests. Only if a higher level interrupt occurs will the processor nest its interrupt service routines. The interrupt service routine must clear the interrupt source and thus remove the request before returning to normal execution. If another interrupt is pending from a different source, it will be recognised and cause another acknowledgement to occur.
Error recovery and control signals
There are three signals associated with error control and recovery. The bus error BERR*, HALT* and RESET* signals can provide information or be used as inputs to start recovery proce-dures in case of system problems.
The BERR* signal is the counterpart of DTACK*. It is used during a bus cycle to indicate an error condition that may arise through parity errors or accessing non-existent memory. If BERR* is asserted on its own, the processor halts normal processing and goes to a special bus error software handler. If HALT* is asserted at the same time, it is possible to rerun the bus cycle. BERR* is removed followed by HALT* one clock later, after which the previous cycle is rerun automatically. This is useful to screen out transient errors. Many designs use external hardware to force a rerun automatically but will cause a full bus error if an error occurs during the rerun.
Without such a signal, the only recourse is to complete the transfer, generate an immediate non-maskable interrupt and let a software handler attempt to sort out the mess! Often the only way out is to reset the system or shut it down. This makes the system extremely intolerant of signal noise and other such transient errors.
The RESET* and HALT* signals are driven low at power-up to force the MC68000 into its power-up sequence. The operation takes about 100 ms, after which the signals are negated and the processor accesses the RESET vector at location 0 in memory to fetch its stack pointer and program counter from the two long words stored there.
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