assembly - Encoding ADC EAX, ECX - 2 different ways to encode? (arch x86) -

i'm looking through intel instruction set manual, , looks there 2 different forms of adc match/encode adc eax, ecx follows:

adc r/m32, r32  (11 /r , encodes 11c8) 

or

adc r32, r/m32  (13 /r, encodes 13c1) 

my question (given did math correctly), 11c8 , 13c1 equivalent? factors assembler consider in selecting 1 encoding on another? question perspective of implementing assembler, question in general, not particular hypothetical instruction.

if it's lengthy answer, please point me in right direction attempts @ googling failed.

this redundancy of instruction encoding. architecture use multiple parameters in instruction has this.

think of risc architecture have add rx, ry, rz assigns sum of ry , rz rx can encode add rx, ry, rz or add rx, rz, ry, they'll equivalent.

in x86 (normally) have 2 parameters each instruction can select direction between them since can store or read memory. if don't use memory can choose direction between 2 registers, there 2 encoding ways

you can use identify compilers/assemblers. assemblers can choose encoding use. in gas can use .s suffix force emit alternate encoding

10 de   adcb   %bl,%dh 12 f3   adcb.s %bl,%dh