20150715 - 1536-2 : Assembling From the Nothing


Started bringing up a limited subset x86-64 assembler. The full x86-64 opcode encoding space is an unfortunate beast of complexity which I'd like to avoid. So I did...

Compromises
This prototype sticks to only exactly 4-byte or 8-byte instructions (8-byte only if the instruction contains a 32-bit immediate/displacement). The native x86-64 opcodes are prefix padded to fill the full 4-byte word. Given that x86-64 CPUs work in chunks of 16-bytes of instruction fetch, this makes it easy to maintain branch alignment visually in the code. Since x86-64 float opcodes are natively 4-bytes without the REX prefix, I'm self limiting to only 8 registers for this assembler, which is good enough for the intended usage. I'm not doing doubles and certainly not wasting time on vector instructions (have an attached GPU for that!). Supported opcode forms in classic Intel syntax,

op;
op reg;
op reg,reg;
op reg,imm32;
op reg,[reg];
op reg,[reg+imm8];
op reg,[reg+imm32];
op reg,[imm32];
op reg,[reg+reg]; <- For LEA only.


This is a bloody ugly list which needed translation into some kind of naming in which "op" changes based on the form. I borrowed some forthisms: @ for load, ! for store. Then added ' for imm8, " for imm32, and # for RIP relative [imm32]. A 32-bit ADD and LEA ends up with this mess of options (note . pushes word value on the stack, so A. pushes 0 for EAX in this context, and , pushes a hex number, and / executes the opcode word which assembles the instruction to the current assembly write position),

A.B.+/ .......... add eax,ebx;
A.1234,"+/ ...... add eax,0x1234;
A.B.@+/ ......... add eax,[rbx];
A.B.12,'@+/ ..... add eax,[rbx+0x12];
A.B.1234,"@+/ ... add eax,[rbx+0x1234];
A.LABEL.#@+/ .... add eax,[LABEL]; <- RIP relative
A.B.12,'+=/ ..... lea eax,[rbx+0x12];
A.B.C.+=/ ....... lea eax,[rbx+rcx*1];


Then using L to expand from 32-bit operand to 64-bit operand,

A.B.L+/ .......... add rax,rbx;
A.1234,L"+/ ...... add rax,0x1234;
A.B.L@+/ ......... add rax,[rbx];
A.B.12,L'@+/ ..... add rax,[rbx+0x12];
A.B.1234,L"@+/ ... add rax,[rbx+0x1234];
A.LABEL.L#@+/ .... add rax,[LABEL]; <- RIP relative
A.B.12,L'+=/ ..... lea rax,[rbx+0x12];
A.B.C.L+=/ ....... lea rax,[rbx+rcx*1];


Source Example With Google Docs Mockup Syntax Highlighting
Font and colors are not what I'm going for, just enough to get to the next step. This is an expanded example which starts building up enough of an assembler to boot and clear the VGA text screen. Some of this got copied from older projects in which I used "X" instead of "L" to mark the 64-bit operand (just noticed I need to fix the shifts...). I just currently copy from this to a text file which gets included into the boot loader on build.

(Lost Image Here When Minus Went Down)

From Nothing to Something
This starts by semi-self-documenting hand assembled x86 instructions via macros. So "YB8-L'![F87B8948,/]" reads like this,

(1.) Y.B.8-,L'! packed to a word name YB8-L'! with tag characters removed.
(2.) [ which starts the macro.
(3.) F87B8948 which is {48 (REX 64-bit operand), 89 (store version of MOV), 79 (modrm byte: edi,[rbx+imm8]), F8 (-8)}.
(4.) , which pushes the number on the data stack.
(5.) / which after , executes the empty word, which pops the data stack and writes 32-bit to the asm position.
(6.) ] which ends the macro.

Later YB8-L'! with ; appended can be used to assemble that instruction by interpreting the macro.

The first assembled words are $ which pushes the current assembly position on the stack, and $DRP (which is actually a bug which needs to be removed). The $! pops an address from the data stack, and stores the current assembly position to given address. This is later used for instruction build macros which do things like PSH` where the ` results in the dictionary address for the PSH word to be placed on the data stack. The end game is getting to the point where given one of the opcode forms, it is possible to write the following to produce a function which compiles an opcode,

C033403E,^`_;

Which pushes the 4-byte opcode base 0xC033403E, then the opcode name ^ for XOR, then runs the _ macro which assembles this into:

MOV eax,0xC033403E;
JMP X86-RM;


Immediately afterwards it is possible to execute the ^ word (call it) and assemble an XOR instruction. The X86-RM expects to get the REG and RM operands from the data stack with base instruction opcode data in EAX.

Making a Mess to Clean Up
This about concludes the worst part of getting going from nothing, except for the PTSD dreams where people only speak in mixed hex and x86 machine code: FUCOM! REX DA TEST JO. When placed into final context there will be a few KB of source to build an assembler which covers all functionality I need for the rest of the system. At this point I can easily add instructions and a few more of the opcode forms as they are they are needed. And it becomes very easy to write assembly like this,

A.A.^/ Y.B8000,"/ C.1F40,"/ L!REP/

Which is this in Intel syntax,

xor eax,eax; <- set eax to zero
mov edi,0xB8000; <- VGA text memory start address
mov ecx,0x1F40; <- 80x50 times two bytes per character
cld;
rep storq; <- using old CISC style slow form to "do:mov [edi],rax;add rdi,8;dec rcx;jnz do;"