Why are the general purpose registers ordered as they are (eax, ecx, edx, ebx)? For example, for the "inc" instruction the opcodes are:
inc eax - 40
inc ecx - 41
inc edx - 42
inc ebx - 43
Is there a reason why they are ordered that way?
4
votes
Not sure why you think they have to be in any certain order unless of course they're pushed onto a stack. Additionally it depends what kind of increment you do, "word", "byte" etc...The destination storage can be either memory location or registers.
– EKet
As it is EAX, EBX etc. those are 32Bit Registers... And well, except the ebx, they seem to be alphabetic, no?
– 0x90
2 Answers
4
votes
The odd placement of (E)BX is probably due to the way that the 8086 evolved from the 8080.
The 8080 has an accumulator (A) and 6 general-purpose registers B, C, D, E, H and L, where B/C, D/E and H/L can be used together in pairs, and in particular H/L can be used as an address for memory access. The 8086 was designed so that existing 8080 code could be easily translated to it; I guess it seemed logical to map the registers in the following order:
8080 register A -> 8086 internal register 0
B,C -> 1
D,E -> 2
H,L -> 3
SP -> 4
As noted in another answer, AX, BX, CX and DX in the 8086 are not just arbitrary names for 4 general-purpose registers - they have mnemonic meanings for the special functions that those registers have: "accumulator", "base", "count" and "data". Given the above mapping, it makes sense to assign the "accumulator" function to internal register 0, and the "base" function to internal register 3. (And 8086 internal registers 5, 6 and 7 are BP, SI and DI, which were new functionality.)
Of course, this is really all just slightly informed (see here for example) speculation - only the 8086 designers know for sure...
0
votes
