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3.3.1
16-Bit Bus Memory and I/O Requirements
A 16-bit bus has certain assumptions that must be met to operate properly. Memory used to store
instruction operands (i.e., the program) and immediate data must be 16 bits wide. Instruction
prefetch bus cycles require that both banks be used. The lower bank contains the even bytes of
code and the upper bank contains the odd bytes of code.
Memory used to store interrupt vectors and stack data must be 16 bits wide. Memory address
space between 0H and 3FFH (1 Kbyte) holds the starting location of an interrupt routine. In re-
sponse to an interrupt, the BIU fetches two consecutive, even-addressed words from this 1 Kbyte
address space. Stack pushes and pops always write or read even-addressed word data.
3.3.2
8-Bit Bus Memory and I/O Requirements
An 8-bit bus interface has no restrictions on implementing the memory or I/O interfaces. All
transfers, bytes and words, occur over the single 8-bit bus. Operations requiring word transfers
automatically execute two consecutive byte transfers.
3.4
BUS CYCLE OPERATION
The BIU executes a bus cycle to transfer data between any of the integrated units and any external
memory or I/O devices (see Figure 3-6). A bus cycle consists of a minimum of four CPU clocks
known as "T-states." A T-state is bounded by one falling edge of CLKOUT to the next falling
edge of CLKOUT (see Figure 3-7). Phase 1 represents the low time of the T-state and starts at the
high-to-low transition of CLKOUT. Phase 2 represents the high time of the T-state and starts at
the low-to-high transition of CLKOUT. Address, data and control signals generated by the BIU
go active and inactive at different phases within a T-state.
BUS INTERFACE UNIT
3-7
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