rasm2 giving different output each time for x64

Question

Disassembling is great but when trying to re-assemble to opcode it gives different results each time. Example:

C:\>rasm2 -a x86 -b 64 -d "48898D80FEFFFF"  
mov qword [rbp - 0x180], rcx  
C:\>rasm2 -a x86 -b 64 "mov qword [rbp - 0x180], rcx"  
49898d80feffff  

See the difference? The first byte has been changed and it keep the same behavior each time. Another issue is, I couldn't get it to assemble this instruction with 8 bytes it only gives me 4 bytes:

C:\>rasm2 -a x86 -b 64 -d "C7458C0000803F"  
mov dword [rbp - 0x74], 0x3f800000  
C:\>rasm2 -a x86 -b 64 "mov dword [rbp - 0x74], 0x3f800000"  
41c67400

But when I remove the the bits flag and set the registers to 32 bit it gives me the correct opcodes!

C:\>rasm2 -a x86 "mov dword [ebp - 0x74], 0x3f800000"  
c7458c0000803f

Why is it doing this and what did I do wrong?

GitHub issue link for future reference.

Answer 1

This was a bug, and the issue that was opened by OP is now fixed on radare2 Master and on the latest releases.

$ rasm2 -a x86 -b 64 -d "48898D80FEFFFF"
mov qword [rbp - 0x180], rcx
$ rasm2 -a x86 -b 64 "mov qword [rbp - 0x180], rcx"
48898d80feffff

$ rasm2 -a x86 -b 64 -d "C7458C0000803F"
mov dword [rbp - 0x74], 0x3f800000
$ rasm2 -a x86 -b 64 "mov dword [rbp - 0x74], 0x3f800000"
c7458c0000803f

The fix can be found in this link.