Editing Ultimate 64 Motherboard (section)

== Turbo mode ==

This is the part most often described wrongly, so it is worth stating precisely. The '''CPU and memory subsystem run at 48 MHz''' on the Ultimate 64 and '''64 MHz''' on the Elite-II. The turbo setting does not change that clock — it determines '''how many of the available time slots are given to the CPU'''.<ref name="doc" />

Selectable CPU speeds:<ref name="doc" />

* '''Ultimate 64''' — 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 20, 24, 32, 40, 48 MHz
* '''Elite-II''' — 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20, 24, 32, 40, 64 MHz

Badline timing can be disabled to free more CPU cycles, '''but the badline setting has no influence on the BA signal at the cartridge port'''. Accesses to external devices therefore still run into the badline limitation — and so do accesses to the SID registers when the physical SID sockets are enabled. That is the real reason a turbo setting can fail to deliver the speed-up you expect.<ref name="doc" />

=== SuperCPU-compatible registers ===

The Ultimate 64 implements registers that mirror the [[SuperCPU and Turbo Accelerators|SuperCPU]]'s, so that existing software can detect and control turbo:<ref name="doc" />

{| class="wikitable styled-table" style="width:75%; text-align:left;"
! Address !! Function
|-
| $D030 (53296) || Turbo Enable Bit. Write 0 = 1 MHz with badlines; 1 = use the menu settings. Read 0 = turbo off, 1 = turbo on.
|-
| $D031 (53297) || U64 Turbo Control. Bits 0–3 CPU speed index; bit 7 badline timing (0 = enabled, 1 = disabled).
|-
| $D07A (53370) || SuperCPU-compatible software speed select — normal.
|-
| $D07B (53371) || SuperCPU-compatible software speed select — turbo.
|-
| $D0BC (53436) || SuperCPU mode detect register.
|}

$D031, $D07A, $D07B and $D0BC read $FF unless the turbo mode selector is set to "U64 Turbo Registers" or "Turbo Enable Bit".<ref name="doc" />