The Intel x86 instruction set

Special/general purpose registers

eax, ebx, ecx, edx - 32 bit
ax is low-order 16 bits of eax
ah and al are the upper and lower 8 bits of ax

esi and edi - source and destination index registers, si and di are the lower 16 bits
esi and edi have special roles in memory-to-memory move operations

esp - the stack pointer, sp is the low-order 16 bits
ebp - the base pointer, same role as fp in the MIPS CPU

eip - the instruction pointer (manipulated by branch/jump instructions)
eflags- the flags register (not manipulated as a whole, but individual bits
have names: CF - carry flag, PF - parity flag, ZF - zero flag, SF - sign flag,
DF - direction flag, OF - overflow flag; CF, OF, PF, SF, ZF are set automatically
by instructions affecting EAX.

Operands
The x86 supports a wide variety of values as direct operands of instructions: register, immediate, 
memory direct, or register indirect.

Operands may be 8, 16, or 32 bits; confusingly in some assemblers these are termed byte, word, and 
doublewords while in others they are byte, word, and long operands.

Assemblers

There are at least two families of assemblers for the x86, Intel-style and AT&T style, and they have radically different syntax. See the 80386 section of the GNU assembler manual. The Jones forth system is written in gnu assembler which uses the AT&T style. Some things to watch for

• They have different operand orders. AT&T style puts the destination on the right, Intel on the left.
• AT&T indicates immediate operands with preceding $, register operands with %. Intel style delimits neither.
• AT&T uses offset(register) notation for memory operands where Intel uses [register + offset]. Offset may be negative in both cases.

These differences can make it tricky to read gnu code and try to figure out what it means from the Intel documentation which assumes Intel style.

Instruction styles

Many Intel instructions have %eax as an implicit destination register, e.g. push and pop. Other instructions have other registers as implicit sources or destinations. We'll try to get a sense of this reading the jonesforth code.