i845 Chipset

The fact that system builders were obliged to use expensive DRDRAM – by virtue of the absence of any Pentium 4 chipsets supporting conventional SDRAM – had been an issue ever since the Pentium 4’s launch at the end of 2000. The situation changed during the course of 2001, with chipmakers SiS and VIA both releasing Pentium 4 chipsets with DDR SDRAM support. Although this was a move of which Intel disapproved, it did have the effect of boosting the appeal of the Pentium 4, whose sales hitherto had been disappointing.

In the summer of 2001 Intel eventually gave in to market pressures and released their 845 chipset – previously codenamed Brookdale – supporting Pentium 4 systems’ use of PC133 SDRAM. Whilst the combination of i845 and PC133 SDRAM meant lower prices – given that the speed of the memory bus was about three times slower than that of the Pentium 4 system bus – it also meant significantly poorer performance than that of an i850/DRDRAM based system. The reason the i845 didn’t support faster DDR SDRAM at this time was apparently because they were prevented from allowing this until the start of the following year by the terms of a contract they’d entered into with Rambus, the inventors of DRDRAM.

Sure enough, at the beginning of 2002 re-released of the i845 chipset. The new version – sometimes being referred to as i845D – differs from its predecessor only in respect of its memory controller, which now supports PC1600 and PC2100 SDRAM – sometimes referred to as DDR200 and DDR266 respectively – in addition to PC133 SDRAM. It had reportedly been Intel’s original intention for the i845 chipset to support only DDR200 SDRAM – capable of providing a maximum bandwidth of 1600MBps. However, the boom in the use of DDR SDRAM – and the consequent dramatic fall in prices – caused a rethink and the subsequent decision to extend support to DDR266 (maximum bandwidth 2100MBps). The fact that the company was prepared to make this decision even though it was bound to adversely impact the market share of its i850 chipset appears to indicate that the company’s apparent infatuation with DRDRAM is well and truly over.

The 400MHz system bus of the i845 solution enables up to 3.2GBps of memory bandwidth to Pentium 4 processor. Compare this with the up to 1 GBps of data transfer possible from PC133 SDRAM and it is clear why faster DDR SDRAM makes such a difference to overall system performance. Its 1.5V 4x AGP interface with provides over 1 GBps of graphics bandwidth. Other features of the i845 chipset include an 4x AGP interface, 133MBps to the PCI, support for four USB ports, six-channel audio, a generally unused LAN connect interface, dual ATA-100 controllers and CNR support.

The i845 is Intel’s first chipset to use a Flip Chip BGA packaging for the chip itself. This improves heat conductivity between the Memory & Controller Hub (MCH) and its heatsink which is required for proper operation. It is also the first MCH built using a 0.18-micron process; earlier versions have been 0.25-micron. The smaller die allows another first – the incorporation of a Level 3-like write cache, significantly increasing the speed at which the CPU is able to write data. It is expected that the transition to 0.13-micron MCH/Northbridges will enable this idea to be further developed, to the point where chipsets include much larger, genuine Level 3 caches on the MCH itself. The i845 further capitalises on the performance advantage realised by its high-speed write cache by the provision of deep data buffers. These play an important role in assisting the CPU and write cache to sustain its high data throughput levels.

A number of newer versions of the i845 chipset were subsequently released, all supporting the USB 2.0 interface (which increases bandwidth up to 40 times over the previous USB 1.1 standard):

• The i845G chipset, incorporating a new generation of integrated graphics – dubbed Intel Extreme Graphics – and targeted at the high-volume business and consumer desktop market segments.
• The i845E chipset, which works with discrete graphics components
• The i845GL chipset, designed for Celeron processor-based PCs.

i845GE chipset

The i845GE chipset was designed and optimised to support Hyper-Threading, Intel’s innovative technology that achieves significant performance gains by allowing a single processor to be treated as two logical processors. Whilst not the first i845 chipset to support HT technology, it was the first in which that support was actually implemented, being launched at the same time as the first Intel’s first HT-enabled desktop CPU, the 3.06GHz Pentium 4 unveiled in late 2002.

As well as supporting a faster, 266MHz version of Intel’s Extreme Graphics core, the i845GE also supports a system bus speed of either 400 or 533MHz, up to DDR333 main memory and offers maximum display (digital CRT or TV) flexibility through an AGP4x connector.

The i845PE and i845GV chipsets are lower-spec variants of the i845GE, the former having no integrated graphics and the latter limiting both the Intel Extreme Graphics core and main memory support to DDR266 SDRAM.

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