Gekko

Gekko is the GameCube's CPU. It is a PowerPC 750 variant including a primitive implementation of SIMD in the form of a set of special 'paired singles' instructions (incompatible with AltiVec), which seems to have been created specifically for Nintendo.

Some later general-sales PowerPC 750 variants are known to have these instructions accessible when flipping some undocumented bits in HID (Hardware-Implementation Dependent) registers - the same way they are enabled on Gekko. A leftover paragraph in the PowerPC 750CX user's manual references the HID2 register, present on Gekko but otherwise undocumented for the 750CX, indicating the two chips may have been developed simultaneously, with the 750CX as a general-sales version with the Nintendo-specific instructions removed from documentation. The successor Broadway CPU also received a general-sales version in the form of the 750CL, however in this case the PS instructions were opened for all to use.

Some 750CX CPUs are known to be pin-compatible with Gekko, and a swap could theoretically be performed.

Versions

PVR_GEKKO_DD10 0x70000100

PVR_GEKKO_DD20_INTENDED PVR_750_DD10 // Apparently, actual Gekko DD2.0 had the DD1.0 PVR written by mistake.

PVR_GEKKO_DD23 0x00083213

PVR_GEKKO_DD23a 0x00083203

PVR_GEKKO_DD23b PVR_GEKKO_DD23 // why??

PVR_GEKKO_DD23e PVR_GEKKO_DD23

PVR_GEKKO_DD23ei PVR_GEKKO_DD23a

PVR_GEKKO_DD24 0x00083204

PVR_GEKKO_DD24e 0x00083214

PVR_GEKKO_DD40 0x00083410

// 2.3=2.3b=2.3e, 2.3a=2.3ei

A Gekko DD1 or 2 CPU (unknown which due to the above-mentioned PVR errata) has been successfully swapped onto a retail GameCube board, with the system booting up and reporting DD1 or 2 in the clkspeed.dol diagnostics tool.