ARM.SoC.Architecture

The other signals are dedicated to more esoteric

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Unformatted text preview: cycle. ARM7TDMI 251 Table 9.1 ARM7TDMI cycle types. mreq 0 0 1 1 seq 0 1 0 1 Cycle N S I Use Non-sequential memory access Sequential memory access Internal cycle - bus and memory inactive Coprocessor register transfer - memory inactive c mas[1:0] encode the memory access size, indicating whether the access is for a byte, half-word or word quantity. bl[3:0] are externally controlled enables on latches on each of the four bytes on the data input bus. These make interfacing memories which are less than 32 bits wide easier. The signals which indicate the type of the memory cycle, mreq and seq, are issued early to give the memory control logic as long as possible to decide how to handle the memory access. The interpretation of the four possible combinations of values on these two signals is given in Table 9.1. When a sequential cycle follows a non-sequential cycle, the address will be that of the non-sequential cycle plus one word (four bytes); where the sequential cycle follows an internal or coprocessor register transfer cycle, the address will be unchanged from the preceding cycle. In a typical memory organization the incrementing case can be used, together with information about the preceding address, to prepare the memory for a fast sequential access and where the address remains the same this can be exploited to start a full memory access in the preceding cycle (since neither the internal nor the coprocessor register transfer cycles use the memory). The timing of the critical interface signals is illustrated in Figure 9.3 on page 252. The use of these signals and the design of the memory interface logic was discussed further in Section 8.1 on page 208, where specific examples are given. MMU interface The interface signals to the MMU provide information which is used to control access to areas of memory. The trans (translation control) signal indicates whether the processor is in a user (trans = 0) or privileged (trans = 1) mode so that some areas of memory can be restricted to supervisor-only access and, where appropriate, di...
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