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MD80C86-2B Datasheet(PDF) 9 Page - Intersil Corporation |
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MD80C86-2B Datasheet(HTML) 9 Page - Intersil Corporation |
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9 / 35 page  9 FN2957.3 January 9, 2009 Word (16-bit) operands can be located on even or odd address boundaries and are thus, not constrained to even boundaries as is the case in many 16-bit computers. For address and data operands, the least significant byte of the word is stored in the lower valued address location and the most significant byte in the next higher address location. The BIU automatically performs the proper number of memory accesses; one, if the word operand is on an even byte boundary and two, if it is on an odd byte boundary. Except for the performance penalty, this double access is transparent to the software. The performance penalty does not occur for instruction fetches; only word operands. Physically, the memory is organized as a high bank (D15-D8) and a low bank (D7-D0) of 512k bytes addressed in parallel by the processor’s address lines. Byte data with even addresses is transferred on the D7-D0 bus lines, while odd addressed byte data (A0 HIGH) is transferred on the D15-D8 bus lines. The processor provides two enable signals, BHE and A0, to selectively allow reading from or writing into either an odd byte location, even byte location, or both. The instruction stream is fetched from memory as words and is addressed internally by the processor at the byte level as necessary. In referencing word data, the BlU requires one or two memory cycles depending on whether the starting byte of the word is on an even or odd address, respectively. Consequently, in referencing word operands performance can be optimized by locating data on even address boundaries. This is an especially useful technique for using the stack, since odd address references to the stack may adversely affect the context switching time for interrupt processing or task multiplexing. Certain locations in memory are reserved for specific CPU operations (see Figure 2). Locations from address FFFF0H through FFFFFH are reserved for operations including a jump to the initial program loading routine. Following RESET, the CPU will always begin execution at location FFFF0H where the jump must be located. Locations 00000H through 003FFH are reserved for interrupt operations. Each of the 256 possible interrupt service routines is accessed through its own pair of 16-bit pointers (segment address pointer and offset address pointer). The first pointer, used as the offset address, is loaded into the lP and the second pointer, which designates the base address is loaded into the CS. At this point, program control is transferred to the interrupt routine. The pointer elements are assumed to have been stored at the respective places in reserved memory prior to occurrence of interrupts. Minimum and Maximum Operation Modes The requirements for supporting minimum and maximum 80C86 systems are sufficiently different that they cannot be met efficiently using 40 uniquely defined pins. Consequently, the 80C86 is equipped with a strap pin (MN/MX) which defines the system configuration. The definition of a certain subset of the pins changes, dependent on the condition of the strap pin. When the MN/MX pin is strapped to GND, the 80C86 defines pins 24 through 31 and 34 in maximum mode. When the MN/MX pin is strapped to VCC, the 80C86 generates bus control signals itself on pins 24 through 31 and 34. The minimum mode 80C86 can be used with either a multiplexed or demultiplexed bus. This architecture provides the 80C86 processing power in a highly integrated form. The demultiplexed mode requires two 82C82 latches (for 64k addressability) or three 82C82 latches (for a full megabyte of addressing). An 82C86 or 82C87 transceiver can also be used if data bus buffering is required (see Figure 6A.) The 80C86 provides DEN and DT/R to control the transceiver, and ALE to latch the addresses. This configuration of the minimum mode provides the standard demultiplexed bus structure with heavy bus buffering and relaxed bus timing requirements. The maximum mode employs the 82C88 bus controller (see Figure 6B). The 82C88 decodes status lines S0, S1 and S2, and provides the system with all bus control signals. Moving the bus control to the 82C88 provides better source and sink current capability to the control lines, and frees the 80C86 pins for extended large system features. Hardware lock, queue status, and two request/grant interfaces are provided by the 80C86 in maximum mode. These features allow coprocessors in local bus and remote bus configurations. Bus Operation The 80C86 has a combined address and data bus commonly referred to as a time multiplexed bus. This technique provides the most efficient use of pins on the processor while permitting the use of a standard 40 lead package. This “local bus” can be buffered directly and used throughout the system with address latching provided on memory and I/O modules. In addition, the bus can also be demultiplexed at the processor with a single set of 82C82 address latches if a standard non-multiplexed bus is desired for the system. Each processor bus cycle consists of at least 4 CLK cycles. These are referred to as t1, t2, t3 and t4 (see Figure 3). The address is emitted from the processor during t1 and data transfer occurs on the bus during t3 and t4. t2 is used primarily for changing the direction of the bus during read operations. In the event that a “NOT READY” indication is given by the addressed device, “Wait” states (tW) are inserted between t3 and t4. Each inserted wait state is the same duration as a CLK cycle. Periods can occur between 80C86 driven bus cycles. These are referred to as idle” states (TI) or inactive CLK cycles. The processor uses these cycles for internal housekeeping and processing. During t1 of any bus cycle, the ALE (Address Latch Enable) signal is emitted (by either the processor or the 82C88 bus controller, depending on the MN/MX strap). At the trailing 80C86 |
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