I thank Brett K. Heath for kindly providing this information from a Motorola Advance Information Datasheet for the SN74LS738/MC6883. To the best of my knowledge this information is accurate, but it is preliminary information, so it may be erroneous.

The S0-S1 outputs from the 6883 SAM are provided to simplify decoding memory blocks by function. There are two memory maps available, selected by the SAM control bit TY. TY is cleared by a write to FFDE, and set by a write to FFDF (the data bus is ignored).

I'm assuming the CoCo uses TY=0 which corresponds to a mixed RAM/ROM memory map but will also include the decoding for TY=1 which is an all RAM memory map.

The output of S0-S2 (S0 is ls bit on pin 27, S1 on pin 28, S2 on pin 25) depends not only upon the Address bus and the setting of TY but also the value of R/W* on that cycle.

Memory Map for TY=0

ADDRESS		R/W*	S	Intended Use
FFF2-FFFF	x	2	MC6809E Reset and Interrupt vectors
FFE0-FFF1	x	2	Reserved for MPU enhancements
FFC0-FFDF	x	7	SAM control registers
FF60-FFBF	x	7	Reserved for contro register enhancements
FF40-FF5F	x	6	I/O2 to subdivide use A0-A4
FF20-FF3F	x	5	I/O1 "			  "
FF00-FF1F	x	4	I/O0 "			  "
C000-FEFF	x	3	ROM2 external cartridge ROM.
A000-BFFF	x	2	ROM1 internal ROM, note that 6809E vector 
				addresses select this ROM.
8000-9FFF	x	1	ROM0 internal ROM.
0000-7FFF	0	7	RAM shared by MPU and VDG
0000-7FFF	1	0	"			"


Memory Map for TY=1

ADDRESS		R/W*	S	Intended Use
FFF2-FFFF	x	2	MC6809E Reset and Interrupt vectors
FFE0-FFF1	x	2	Reserved for MPU enhancements
FFC0-FFDF	x	7	SAM control registers
FF60-FFBF	x	7	Small ROM, boot load program and vectors 
FF40-FF5F	x	6	I/O2 to subdivide use A0-A4
FF20-FF3F	x	5	I/O1 "			  "
FF00-FF1F	x	4	I/O0 "			  "
0000-FEFF	1	0	RAM shared by MPU and VDG
C000-FEFF	0	3
A000-BFFF	0	2
8000-9FFF	0	1
0000-7FFF	0	7