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diff --git a/subx_addressing_modes.md b/subx_addressing_modes.md new file mode 100644 index 00000000..cdb056ee --- /dev/null +++ b/subx_addressing_modes.md @@ -0,0 +1,125 @@ +[The Readme](Readme.md) describes SubX notation with some details hidden +behind _syntax sugar_ -- local rewrite rules that make programming in SubX +less error-prone. However, much low-level SubX (before the syntax sugar is +implemented) is written without syntax sugar. This document describes some +details of the syntax sugar: how the reg/mem operand is translated into +arguments. + +## How x86 instructions compute operands + +The [Intel processor manual](http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf) +is the final source of truth on the x86 instruction set, but it can be +forbidding to make sense of, so here's a quick orientation. You will need +familiarity with binary numbers, and maybe a few other things. Email [me](mailto:mu@akkartik.com) +any time if something isn't clear. I love explaining this stuff for as long as +it takes. The bad news is that it takes some getting used to. The good news is +that internalizing the next 500 words will give you a significantly deeper +understanding of your computer. + +The reg/mem operand can be specified by 1-7 arguments, each ranging in size +from 2 bits to 4 bytes. The key argument that's always present for reg/mem +operands is `/mod`, the _addressing mode_. This is a 2-bit argument that can +take 4 possible values, and it determines what other arguments are required, +and how to interpret them. + +* If `/mod` is `3`: the operand is in the register described by the 3-bit + `/rm32` argument. + +* If `/mod` is `0`: the operand is in the address provided in the register + described by `/rm32`. That's `*rm32` in C syntax. + +* If `/mod` is `1`: the operand is in the address provided by adding the + register in `/rm32` with the (1-byte) displacement. That's `*(rm32 + /disp8)` + in C syntax. + +* If `/mod` is `2`: the operand is in the address provided by adding the + register in `/rm32` with the (4-byte) displacement. That's `*(/rm32 + + /disp32)` in C syntax. + +In the last three cases, one exception occurs when the `/rm32` argument +contains `4`. Rather than encoding register `esp`, it means the address is +provided by three _whole new_ arguments (`/base`, `/index` and `/scale`) in a +_totally_ different way (where `<<` is the left-shift operator): + + ``` + reg/mem = *(base + (index << scale)) + ``` + +(There are a couple more exceptions ☹; see [Table 2-2](modrm.pdf) and [Table 2-3](sib.pdf) +of the Intel manual for the complete story.) + +Phew, that was a lot to take in. Some examples to work through as you reread +and digest it: + +1. To read directly from the `eax` register, `/mod` must be `3` (direct mode), + and `/rm32` must be `0`. There must be no `/base`, `/index` or `/scale` + arguments. + +1. To read from `*eax` (in C syntax), `/mod` must be `0` (indirect mode), and + the `/rm32` argument must be `0`. There must be no `/base`, `/index` or + `/scale` arguments (Intel calls the trio the 'SIB byte'.). + +1. To read from `*(eax+4)`, `/mod` must be `1` (indirect + disp8 mode), + `/rm32` must be `0`, there must be no SIB byte, and there must be a single + displacement byte containing `4`. + +1. To read from `*(eax+ecx+4)`, one approach would be to set `/mod` to `1` as + above, `/rm32` to `4` (SIB byte next), `/base` to `0`, `/index` to `1` + (`ecx`) and a single displacement byte to `4`. (What should the `scale` bits + be? Can you think of another approach?) + +1. To read from `*(eax+ecx+1000)`, one approach would be: + - `/mod`: `2` (indirect + disp32) + - `/rm32`: `4` (`/base`, `/index` and `/scale` arguments required) + - `/base`: `0` (eax) + - `/index`: `1` (ecx) + - `/disp32`: 4 bytes containing `1000` + +## Putting it all together + +Here's an example showing these arguments at work: + +<img alt='apps/ex3.subx' src='html/ex3.png'> + +This program sums the first 10 natural numbers. By convention I use horizontal +tabstops to help read instructions, dots to help follow the long lines, +comments before groups of instructions to describe their high-level purpose, +and comments at the end of complex instructions to state the low-level +operation they perform. Numbers are always in hexadecimal (base 16) and must +start with a digit ('0'..'9'); use the '0x' prefix when a number starts with a +letter ('a'..'f'). I tend to also include it as a reminder when numbers look +like decimal numbers. + +--- + +I recommend you order arguments consistently in your programs. SubX allows +arguments in any order, but only because that's simplest to explain/implement. +Switching order from instruction to instruction is likely to add to the +reader's burden. Here's the order I've been using after opcodes: + +``` + |<--------- reg/mem --------->| |<- reg/mem? ->| +/subop /mod /rm32 /base /index /scale /r32 /displacement /immediate +``` + +--- + +Try running this example now: + +```sh +$ ./bootstrap translate init.linux apps/ex3.subx -o apps/ex3 +$ ./bootstrap run apps/ex3 +$ echo $? +55 +``` + +If you're on Linux you can also run it natively: + +```sh +$ ./apps/ex3 +$ echo $? +55 +``` + +These details should now be enough information for reading and modifying +low-level SubX programs. |