Specifications

A Minimal PowerPCª Boot Sequence for

Executing Compiled C Program

PowerPC Processor Initialization

The procedure for initializing a pair of BAT registers is as follows:

1. Disable the MMU.

2. If modifying a data BAT, execute an

isync

instruction.

3. Initialize the lower portion of the BAT array entry.

4. Initialize the upper portion of the BAT array entry.

5. Execute an

isync

instruction.

6. Re-enable the MMU when all setup is complete.

Unused BAT registers must be invalidated by clearing the Vs and Vp bits in the upper BAT register before

enabling and translating.

For each region of memory to be mapped, an appropriate BL and BEPI must be chosen. The BL Þeld is an

encoding of the length of the block to be mapped. The BEPI Þeld corresponds to the upper bits of the logical

address of a region to be mapped onto physical memory. During address translation, addresses are compared

with the BEPI Þeld to determine if a BAT array hit has occurred.

Next, the BRPN must be chosen to indicate the physical memory onto which the logical region speciÞed by

the BEPI is to be mapped. For many minimal systems where the logical and physical addresses are

equivalent, including the sample located at the end of this document, the BEPI and BRPN will be equal.

Note that the values in the BEPI and BRPN Þelds must have at least as many low order zeroes as the BL has

ones. Otherwise, the results are boundedly undeÞned. The possible BL encodings are shown in Table 3.

The Vs and Vp bits in the upper BAT register, along with the PP bits in the lower BATs, specify the access

controls for the memory region. A region may be marked valid for supervisor mode, valid for user mode, or

valid for both modes; Table 4 shows these options.

Table 3. BL Encodings

Block Size BL Encoding

128 Kbytes 000 0000 0000

256 Kbytes 000 0000 0001

512 Kbytes 000 0000 0011

1 Mbyte 000 0000 0111

2 Mbytes 000 0000 1111

4 Mbytes 000 0001 1111

8 Mbytes 000 0011 1111

16 Mbytes 000 0111 1111

32 Mbytes 000 1111 1111

64 Mbytes 001 1111 1111

128 Mbytes 011 1111 1111

256 Mbytes 111 1111 1111