User’s Manual

IBM PowerPC 750GX and 750GL RISC Microprocessor

8.3.2.1 Address-Bus Parity

The 750GX always generates 1 bit of correct odd-byte parity for each of the 4 bytes of address when a valid address is on the bus. The calculated values are placed on the AP[0–3] outputs when the 750GX is the address-bus master. If the 750GX is not the master and TS and GBL are asserted together (qualified condition for snooping memory operations), the calculated values are compared with the AP[0–3] inputs. If there is an error, and address-parity checking is enabled (HID0[EBA] set to 1), a machine-check exception is gener- ated. An address-bus parity error causes a checkstop condition if MSR[ME] is cleared to 0. For more information about checkstop conditions, see Chapter 4, Exceptions, on page 151.

8.3.2.2 Address Transfer Attribute Signals

The transfer attribute signals include several encoded signals such as the transfer type (TT[0–4]) signals, transfer burst (TBST) signal, transfer size (TSIZ[0–2]) signals, write-through (WT), and cache inhibit (CI). Section 7.2.4, Address Transfer Attribute Signals, on page 255 describes the encodings for the address transfer attribute signals.

Transfer Type (TT[0–4]) Signals

Snooping logic should fully decode the transfer type signals if the GBL signal is asserted. Slave devices can sometimes use the individual transfer type signals without fully decoding the group. For a complete description of the encoding for TT[0–4], see Table 8-1and Table 8-2on page 295.

Transfer Size (TSIZ[0–2]) Signals

The TSIZ[0–2] signals indicate the size of the requested data transfer as shown in Table 8-1. The TSIZ[0–2] signals can be used along with TBST and A[29–31] to determine which portion of the data bus contains valid data for a write transaction or which portion of the bus should contain valid data for a read transaction. Note that, for a burst transaction (as indicated by the assertion of TBST), TSIZ[0–2] are always set to 0b010.

Therefore, if the TBST signal is asserted, the memory system should transfer a total of eight words (32 bytes), regardless of the TSIZ[0–2] encodings.

Table 8-1. Transfer Size Signal Encodings

 

 

 

 

 

 

 

 

TBST

TSIZ0

TSIZ1

TSIZ2

Transfer Size

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Asserted

0

1

0

8-word burst

 

 

 

 

 

 

 

Negated

0

0

0

8 bytes

 

 

 

 

 

 

 

Negated

0

0

1

1 byte

 

 

 

 

 

 

 

Negated

0

1

0

2 bytes

 

 

 

 

 

 

 

Negated

0

1

1

3 bytes

 

 

 

 

 

 

 

Negated

1

0

0

4 bytes

 

 

 

 

 

 

 

Negated

1

0

1

5 bytes (N/A)

 

 

 

 

 

 

 

Negated

1

1

0

6 bytes (N/A)

 

 

 

 

 

 

 

Negated

1

1

1

7 bytes (N/A)

 

 

 

 

 

 

 

Bus Interface Operation

gx_08.fm.(1.2)

Page 294 of 377

March 27, 2006