Can someone explain this directly assembled x86 JMP opcode?

Answer 1

The 0x66 indicates that the JMP (0xEA) refers to six bytes. The default is refering to 64K (16 bits) in real mode or to 32 bits in protected mode (if I recall well). Having it increased, it also includes the segment descriptor, the index of the segment either in the GDT or the LDT, which means, that this code is making what is traditionally called a "long jump": a jump that cross beyond segments in the x86 architecture. The segment, in this case, points to the second entry on the GDT. If you look before in that program, you'll likely see how the GDT is defined in terms of the segment starting address and length (look in the Intel manual to study the GDT and LDT tables, 32 bit entry describing each segment).

Answer 2

I run into this a bit. Some assemblers will only jump to a LABEL . In this case the person wants to make an absolute jump to a specific hard coded offset. jmp TARGET_ADDRESS won't work I am guessing, so they just put it as bytes to get around this issue.