readme tweaks, mainly
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pommicket
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13 changed files with 177 additions and 84 deletions
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@ -1,7 +1,9 @@
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all: out01 out02 README.html
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out01: in01
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../01/out00
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out02: out01
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out02: out01 in02
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./out01
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%.html: %.md ../markdown
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../markdown $<
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clean:
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rm -f out01 out02 README.html
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116
02/README.md
116
02/README.md
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@ -1,13 +1,15 @@
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# stage 02
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The compiler for this stage is in the file `in01`, an input for our previous compiler.
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The specifics of how this compiler works are in the comments in that file, but here I'll
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So if you run `../01/out00`, you'll get the file `out01`, which is
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this stage's compiler.
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The specifics of how this compiler works are in the comments in `in01`, but here I'll
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give an overview.
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Let's take a look at `in02`, an example input file for this compiler:
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```
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jm
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:-co jump to code
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::hw
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::hw start of hello world
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'H
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'e
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'l
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@ -23,11 +25,12 @@ jm
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'!
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\n
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::he end of hello world
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::co start of code
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//
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// now we'll calculate the length of the hello world string
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// calculate the length of the hello world string
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// by subtracting hw from he.
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//
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im
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--he
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BA
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@ -36,7 +39,7 @@ im
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nA
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+B
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DA put length in rdx
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// okay now we can write it
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// okay now write it
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im
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##1.
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JA set rdi to 1 (stdout)
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@ -54,56 +57,123 @@ im
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sy
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```
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You can try adding more characters to the hello world message, and it'll just work;
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the length of the text is computed automatically!
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We can compile it by running `./out01`. This will produce
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the executable `out02`, which you can run. It prints
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`Hello, world!`.
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This time, commands are separated by newlines instead of semicolons.
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Each line begins with a 2-character command identifier. There are some special identifiers though:
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In this language,
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commands are separated by newlines instead of semicolons.
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Each line begins with a 2-character command.
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All of the commands from the previous compiler are here,
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plus six new ones:
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- `::` marks a *label*
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- `--` outputs a label's (absolute) address
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- `:-` outputs a label's relative address
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- `##` outputs a number
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All other commands work like they did in the previous compiler—if you scroll down in the
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`in01` source file, you'll see the full command table.
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- `//` is for comments
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- `\n\n` does nothing (used for spacing)
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## labels
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Labels are the most important new feature of this language.
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A line like
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```
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::xy
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```
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associates the name `xy` with the address of the next byte of the program.
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In the example program, `hw` is associated with `0x40007d`,
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which is the virtual memory address of the `Hello, world!` data.
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We can then use
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```
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--xy
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```
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to output that address, and
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```
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:-xy
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```
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to output it relative to the current address.
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So now instead of computing how far to jump, we can just jump to a label, e.g.
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```
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jm
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:-xy (use the relative address, because jumps are relative in x86-64)
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```
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And instead of figuring out the address of a piece of data, we can just use its label:
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```
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im
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--xy
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// rax now points to the data at the label "::xy"
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```
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This also lets us compute the length of the hello world string automatically!
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By taking the address of the end of the string (`he`) and subtracting the
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start (`hw`), we get the length in bytes.
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So you can try adding more characters to the hello world message, and it'll just work.
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All labels must be two ASCII characters. The address of each label is stored
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as a 32-bit number in the "label table". This is sort of like the command table—the
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index of the label `xy` is `128 * x + y`. Specifically, the entry for `xy` is at
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`0x420000 + 4 * (128 * x + y)`, since the label table starts at `0x420000`
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and each entry is 4 bytes.
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When we encounter `::xy`, we get the current position in the output file
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(using `lseek`), add the address of the start of the file (`0x400000`),
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and store that in the label table.
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When we encounter `:-xy` or `--xy`, we look up `xy` in the label table,
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and write the address (subtracting the current address for `:-`) to the output file.
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## two passes?
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This compiler actually needs to read through the source code,
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and output an executable, twice.
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This is because a label may be defined *after* it is used, e.g.:
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```
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jm
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:-aa jump forward
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...
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::aa this is where we're jumping to
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...
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```
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In the first pass, the `:-aa` will
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treat `aa` as having an address of 0. Then when
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we get to `::aa`, the address in the label table will be corrected.
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At the end of the first pass, we seek back to the start
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of the input and output files,
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and run the exact same code for the second pass.
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But this time, the correct address of `aa` is used, namely the
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one we calculated in the first pass.
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## other features
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Now instead of writing out each of the 8 bytes making up a number,
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we can just write it in hexadecimal (e.g. `##3c.` for `3c 00 00 00 00 00 00 00`),
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and the compiler will automatically
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extend it to 8 bytes.
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we can just write it in hexadecimal, e.g. `##1c4.` for `c4 01 00 00 00 00 00 00`.
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This is especially nice because we don't need to write numbers backwards
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for little-endianness anymore!
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Numbers cannot appear at the end of a line (this was
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to make the compiler simpler to write), so I'm adding a `.` at the end of
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Numbers cannot appear at the end of a line (this made
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the compiler simpler to write), so I'm adding a `.` at the end of
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each one to avoid making that mistake.
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Anything after a command is treated as a comment;
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additionally `//` can be used for comments on their own lines.
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I decided to implement them as simply as possible:
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I decided to implement this as simply as possible:
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I just added the command `//` to the command table, which outputs the byte `0x90`—this
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means "do nothing" (`nop`) in x86-64.
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Note that this means that the following code will not work as expected:
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means ["do nothing"](https://en.wikipedia.org/wiki/No-op)
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in x86-64.
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Note that the following code will not work as expected:
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```
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im
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// load the value 0x333 into rax
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##333.
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```
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since `0x90` gets inserted between the "load immediate" instruction code, and the immediate.
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since `0x90` gets inserted between the "load immediate" instruction code and the immediate.
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`\n\n` works identically, and lets us space out code a bit. But be careful:
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the number of blank lines must be a multiple of 3!
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## limitations
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Many of the limitations of our previous compilers apply to this one. Also,
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if you use a label without defining it, it uses address 0, rather than outputting
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an error message. This could be fixed: if the value in the label table is 0, and if we are
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an error message. This could be fixed: if the value in the label table is 0 and we are
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on the second pass, output an error message. This compiler was already tedious enough
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to implement, though!
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But thanks to labels, for future compilers at least we won't have to calculate
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24
02/in01
24
02/in01
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@ -3,7 +3,7 @@
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;'i;'n;'0;'2;00 (0x40007d) input filename
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;'o;'u;'t;'0;'2;00 (0x400082) output filename
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;00;00;' ;'n;'o;'t;' ;'r;'e;'c;'o;'g;'n;'i;'z;'e;'d;\n;00;00;00;00;00;00 (0x400088) error message/where we read to
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;00 (0x4000a0) stores which pass we're on (1 for second pass)
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;00 (0x4000a0) stores which pass we're on (0 for first pass, 1 for second pass)
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;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00 (0x4000a8) used for output
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unused padding
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@ -180,11 +180,11 @@ okay it's 0-9
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;+B
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;BA
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okay we now have a digit in RBX
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okay we now have a digit in rbx
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;AR
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;<I;04
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;+B
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;RA store away in RBP
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;RA store away in rbp
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;jm;38;ff;ff;ff continue loop
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unused padding
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@ -195,7 +195,7 @@ unused padding
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;00;00;00;00;00;00;00;00;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00;00;00;00;00;00;00;00;00
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okay we have a full number in RBP, time to write it to the file
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okay we have a full number in rbp, time to write it to the file.
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start by putting it at address 0x4000a8
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;im;a8;00;40;00;00;00;00;00
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;BA
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@ -210,7 +210,7 @@ now write
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;IA
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;im;08;00;00;00;00;00;00;00 write 8 bytes
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;DA
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;im;01;00;00;00;00;00;00;00 write
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;im;01;00;00;00;00;00;00;00 write
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;sy
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;jm;c3;03;00;00 skip to newline
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@ -327,11 +327,11 @@ subtract current address
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;nA;+B
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;RA store relative address in rbp
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now we want to write eax to the output file.
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now we want to write ebp to the output file.
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start by putting it at address 0x4000a8
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;im;a8;00;40;00;00;00;00;00
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;BA
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;AR put relative address in rax
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;AR
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;sd
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now write
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;IA
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;im;04;00;00;00;00;00;00;00 4 bytes
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;DA
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;im;01;00;00;00;00;00;00;00 write
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;im;01;00;00;00;00;00;00;00 write
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;sy
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;jm;66;01;00;00 skip to newline
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@ -368,7 +368,7 @@ it's not a label or a number. let's look it up in the instruction table.
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;BA
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;RA store away address of command text in rbp
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;zA;lb
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;DA number of bytes to write (used for syscall if no error)
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;DA number of bytes to write (used for syscall if command exists)
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;BA
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;zA
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;cm;jn;54;00;00;00 check if # of bytes is 0, if not, skip outputting error
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@ -392,7 +392,7 @@ this is a real command
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;im;01;00;00;00;00;00;00;00 add 1 because we don't want to write the length
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;+B
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;IA address of data to write
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;im;04;00;00;00;00;00;00;00 out file descriptor
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;im;04;00;00;00;00;00;00;00 out file descriptor
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;JA
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;im;01;00;00;00;00;00;00;00 write
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;sy
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@ -1777,7 +1777,7 @@ the formatting changed appropriately.
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;01;90;00;00;00;00;00;00 \n\n
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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@ -6550,7 +6550,7 @@ the formatting changed appropriately.
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;01;90;00;00;00;00;00;00
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;01;90;00;00;00;00;00;00 // comments
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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;00;00;00;00;00;00;00;00
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11
02/in02
11
02/in02
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@ -1,6 +1,6 @@
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jm
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:-co jump to code
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::hw
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::hw start of hello world
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'H
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'e
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'l
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@ -16,11 +16,12 @@ jm
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'!
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\n
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::he end of hello world
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::co start of code
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//
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// now we'll calculate the length of the hello world string
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// calculate the length of the hello world string
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// by subtracting hw from he.
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//
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im
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--he
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BA
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@ -29,7 +30,7 @@ im
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nA
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+B
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DA put length in rdx
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// okay now we can write it
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// okay now write it
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im
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##1.
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JA set rdi to 1 (stdout)
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