ShiftJisは
OEMLeadBytes = [#$81..#$9F, #$E0..#$FC];
if c in OEMLeadBytes then 2byte else 1byte;
EUC-jpは
EUCLeadBytes  = [#$8E,#$8F,#$A1..#$FE];
EUCLeadBytes3 = [#$8F];  //先頭文字が8Fなら合計3byte
if c in EUCLeadBytes then begin
if c in EUCLeadBytes3 then 3byte else 2byte;
end  else 1byte
UTF8は
UTF8LeadBytes = [#$C2..#$DF, #$E0..#$EF, #$F0..#$F4];
UTF8LeadBytes2 = [#$C2..#$DF];
UTF8LeadBytes3 = [#$E0..#$EF];
UTF8LeadBytes4 = [#$F0..#$F4];
if  c in  UTF8LeadBytes then begin
if c in UTF8LeadBytes2 then 2byte
else
if c in UTF8LeadBytes3 then 3byte
else
if c in UTF8LeadBytes3 then 4byte;
end;

var  i: TSSEMODE; s:string;
begin
s:=''; for i := low(SSEname) to High(SSEname) do
if i in d then s := s + ' ' + SSEname[i];

{$ASMMODE intel}
type
TSSEMODE = (sseSSE2, sseSSE3, sseFMA, sseSSE41, sseSSE42, sseAVX, sseAVX2, sseAVX512F, sseAVX512DQ);
TSSEMODEset = set of TSSEMODE;
const SSEname: array [low(TSSEMODE)..High(TSSEMODE)] of string =
('SSE2', 'SSE3', 'FMA', 'SSE41', 'SSE42', 'AVX', 'AVX2', 'AVX512F', 'AVX512DQ');

function GetSSE(): TSSEMODEset;
var iSZ, iDX, iCX, iBX: DWORD;
begin
asm
MOV     EAX,0
CPUID
MOV     iSZ,EAX
MOV     EAX,1
CPUID
MOV     iDX,EDX
MOV     iCX,ECX
MOV     EAX,7
MOV     ECX,0
CPUID
MOV     iBX,EBX
end ['EAX', 'ECX', 'EDX', 'EBX'];
if iSZ < 7 then iBX := 0;
Result := [];
if (iDX and (1 shl 26)) <> 0 then Include(Result, sseSSE2);
if (iCX and (1 shl 0)) <> 0 then Include(Result, sseSSE3);
if (iCX and (1 shl 12)) <> 0 then Include(Result, sseFMA);
if (iCX and (1 shl 19)) <> 0 then Include(Result, sseSSE41);
if (iCX and (1 shl 20)) <> 0 then Include(Result, sseSSE42);
if (iCX and (1 shl 28)) <> 0 then Include(Result, sseAVX);
if (iBX and (1 shl 5)) <> 0 then Include(Result, sseAVX2);
if (iBX and (1 shl 16)) <> 0 then Include(Result, sseAVX512F);
if (iBX and (1 shl 17)) <> 0 then Include(Result, sseAVX512DQ);
end;