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MPC8280 Datasheet(PDF) 290 Page - Freescale Semiconductor, Inc |
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MPC8280 Datasheet(HTML) 290 Page - Freescale Semiconductor, Inc |
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290 / 1386 page  The 60x Bus MPC8280 PowerQUICC II Family Reference Manual, Rev. 1 8-14 Freescale Semiconductor 8.4.3.5 Effect of Alignment on Data Transfers Table 8-6 lists the aligned transfers that can occur to and from the MPC8280. These are transfers in which the data is aligned to an address that is an integer multiple of the size of the data. For example, Table 8-6 shows that 1-byte data is always aligned; however, a 4-byte word must reside at an address that is a multiple of 4 to be aligned. In Figure 8-6, Table 8-6, and Table 8-7, OP0 is the most-significant byte of a word operand and OP7 is the least-significant byte. The MPC8280 supports misaligned memory operations, although they may degrade performance substantially. A misaligned memory address is one that is not aligned to the size of the data being transferred (such as, a word read from an odd byte address). The MPC8280’s processor bus interface supports misaligned transfers within a word (32-bit aligned) boundary, as shown in Table 8-7. Note that the 4-byte transfer in Table 8-7 is only one example of misalignment. As long as the attempted transfer does not cross a word boundary, the MPC8280 can transfer the data to the misaligned address within a single bus transfer (for example, a half-word read from an odd byte-aligned address). It takes two bus transfers to access data that crosses a word boundary. Table 8-6. Aligned Data Transfers Program Transfer Size TSIZ[0–3] A[29–31] Data Bus Byte Lanes D0... ...D31 D32... ...D63 B0 B1 B2 B3 B4 B5 B6 B7 Byte 00 01 0 0 0 OP01 1 OPn: These lanes are read or written during that bus transaction. OP0 is the most-significant byte of a word operand and OP7 is the least-significant byte. —2 2 —: These lanes are ignored during reads and driven with undefined data during writes. —————— 00 01 0 0 1 — OP1 — — — — — — 00 01 0 1 0 — — OP2 — — — — — 00 01 0 1 1 — — — OP3 — — — — 0 0 0 1 1 0 0 — ——— OP4 — —— 0 0 0 1 1 0 1 — ———— OP5 —— 0 0 0 1 1 1 0 — ————— OP6 — 0 0 0 1 1 1 1 — —————— OP7 Half-Word 0 0 1 0 0 0 0 OP0 OP1 — ————— 00 10 0 1 0 — — OP2 OP3 — — — — 0 0 1 0 1 0 0 — ——— OP4 OP5 —— 0 0 1 0 1 1 0 — ————— OP6 OP7 Word 01 00 0 0 0 OP0 OP1 OP2 OP3 — — — — 01 00 1 0 0 — — — — OP4 OP5 OP6 OP7 Double-Word 0 0 0 0 0 0 0 OP0 OP1 OP2 OP3 OP4 OP5 OP6 OP7 |
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