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PPC 7457 performance tweak for new PowerBooks
Authored by: persson on Oct 15, '03 11:37:37AM
The following is taken from the "G3 and G4 Processors and News..." forum in the Mac Achia of the ArsTechnica forums. It sure seems to me that at face value it indicates you should re-disable the BTIC if you want a stable machine!

I was unable to find this described in the errate for the 7457 on Motorola's website.

muscou Smack-Fu Master, in training Registered: September 26, 2003 Posts: 2

posted October 13, 2003 10:37

A few comments about the latest 74x7 errata( see bottom of post)

Does the 1.5% average performance hit suggested by Motorola can be corrobated by somebody on this forum using a few applications.

How can they let a bug like this one go through on what is supposed to be only a die shrink!!

Error No. 20: BTIC must not be enabled by software

Overview:

Enabling the Branch Target Instruction Cache (BTIC) of the Branch Processing Unit (BPU) of the processor will result in the attempted execution of corrupted instructions.

Detailed Description:

The BTIC is a 128-entry, four-way set-associative cache that contains the most recently used branch target instructions (up to four instructions per entry) for b and bc branches. When a taken branch instruction of this type hits in the BTIC, the instructions arrive in the instruction queue a cycle sooner than they would arrive from the instruction cache. Due to an error in the processor, the BTIC may provide corrupted instructions and should not be enabled. The BTIC is enabled in software by setting HID0[BTIC]. The default state of the BTIC is disabled.

Projected Impact: Symptoms of processor failures when HID0[BTIC] is set include unexpected exceptions, including but not limited to instruction cache parity errors. The performance of the processor is impacted if the BTIC is disabled. The actual impact is code-dependent but is anticipated to be between 0% and 3% on typical code streams (compared with. the performance of a processor with a functioning BTIC), with an average of 1.5%.

Work Arounds: None. Do not set HID0[BTIC] = 1.

Projected Solution: Under review.

Link to ars forum quoted above

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