How the TRS-80 Color Computer 1, 2 and 3 High-Speed Pokes Function

When you POKE any number into the CoCo's address range of $FFD6 to $FFD9 (65494 to 65497), you are either CLEARING or SETTING the CPU RATE registers of the 6883/74LS783 or 74LS785 S.A.M. (Synchronous Address Multiplexer) chip <= CoCo 1 and 2 or the G.I.M.E. (Graphics Interrupt Memory Enhancement) chip <= CoCo 3. These registers are as follows:

R1,R0 = CPU RATE REGISTERS
R0 IS CLEARED BY ANY WRITE TO $FFD6 (65494)
R0 IS SET BY ANY WRITE TO $FFD7 (65495)
R1 IS CLEARED BY ANY WRITE TO $FFD8 (65496)
R1 IS SET BY ANY WRITE TO $FFD9 (65497)

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It is important to understand that these are S.A.M. registers, and since the S.A.M. has no data lines connected to it, the actual data written is irrelevant. The S.A.M. registers are simply a series of WRITE ONLY latches or switches. Writing to an even address clears the latch assigned to that address range and writing to an odd address sets the latch assigned to that address range. Each latch occupies two consecutive memory addresses. 

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STUDY THE TABLES BELOW FOR MORE DETAILED INFORMATION:
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=TABLE 1 : CPU RATE REGISTER DESCRIPTIONS              =
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DECIMAL  HEXADECIMAL  WRITE HERE TO:          REGISTER
65494      $FFD6             CLEAR R0 CPU RATE     R0
65495      $FFD7             SET R0 CPU RATE         R0
65496      $FFD8             CLEAR R1 CPU RATE     R1
65497      $FFD9             SET R1 CPU RATE         R1
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*TABLE 2 : CPU RATE REGISTER COMBINATIONS                                        *
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R1 R0          CPU SPEED                   NOTES
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 0   0           .89 Mhz ONLY              THIS IS THE NORMAL OPERATING
                                                    FREQUENCY OF THE COCO 1,2,3
                                                    TDP-100 AND TANO DRAGON.
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 0   1           .89 Mhz / 1.78 Mhz       CPU SPEED IS ADDRESS DEPENDENT. SEE
                                                   TEXT. (THIS IS THE HIGH-SPEED MODE
                                                   OF THE COCO 1,2 TDP-100 AND TANO /
                                                   DRAGON DATA PRODUCTS "DRAGON").
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 1   0          1.78 Mhz ONLY             THIS IS THE HIGH-SPEED MODE OF THE
                                                   CoCo 3, WHICH OPERATES AT THIS
                                                   SPEED 100% OF THE TIME UNDER OS-9
                                                   LEVEL 2. SOME LATER MODEL CoCo 2's
                                                   COULD OPERATE AT THIS SPEED WITH
                                                   LOSS OF VIDEO.
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 1   1           UNUSED                     UNDEFINED STATE.
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