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Writeup, part 2, partially done

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psf 2022-05-21 01:50:46 -07:00
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frustration.rs
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@ -62,18 +62,17 @@ const ADDRESS_SPACE: usize = 65536;
* *
* In most CPUs, this place is called a "register". Registers work like * In most CPUs, this place is called a "register". Registers work like
* variables in a programming language but there are only a few of them * variables in a programming language but there are only a few of them
* (between 1 to 32 depending on your CPU). * (most CPUs have between 1 and 32).
* *
* On 64-bit ARM the registers are named r0, r1, ..., r15. * On 64-bit ARM the registers are named r0, r1, ..., r15.
* On 64-bit Intel they are instead named rax, rbx, ..., etc. * On 64-bit Intel they are instead named rax, rbx, ....
* Just in case those names ring any bells.
* *
* Having immediate access to many registers is quite handy, but it means * Having immediate access to dozens of registers is quite handy, but it means
* many choices are available to the programmer, or more likely, to the * many choices are available to the programmer, or more likely, to the
* compiler. And making good choices is Hard. * compiler. And making good choices is Hard. A lot of work goes into
* * deciding what variable to store in what register ("register allocation") and
* A lot of work goes into deciding what variable to store in what register * when to dump register contents back into memory ("spilling").
* ("register allocation") and when to dump register contents back into
* memory ("spilling").
* *
* Our CPU avoids these problems by not having registers; instead we store * Our CPU avoids these problems by not having registers; instead we store
* numbers in a stack. The CPU can only access the value that was most * numbers in a stack. The CPU can only access the value that was most
@ -86,14 +85,14 @@ const ADDRESS_SPACE: usize = 65536;
#[derive(Debug)] #[derive(Debug)]
struct Stack<const N: usize> { struct Stack<const N: usize> {
mem: [u16; N], mem: [u16; N],
tos: usize // top-of-stack. tos: usize /* top-of-stack */
} }
impl<const N: usize> Stack<N> { impl<const N: usize> Stack<N> {
// Add a number to the stack. /* Add a number to the stack. */
fn push(&mut self, val: u16) { fn push(&mut self, val: u16) {
self.tos = (self.tos.wrapping_add(1)) & (N - 1); self.tos = (self.tos.wrapping_add(1)) & (N - 1);
/* This stack is fixed-sized and can hold N values. /* This stack is fixed-size and can hold N values.
* *
* When a fixed-size stack fills up, there is a failure case * When a fixed-size stack fills up, there is a failure case
* (stack overflow) that must be handled somehow. * (stack overflow) that must be handled somehow.
@ -106,7 +105,7 @@ impl<const N: usize> Stack<N> {
self.mem[self.tos] = val; self.mem[self.tos] = val;
} }
// Return the most recently pushed number. /* Return the most recently pushed number. */
fn pop(&mut self) -> u16 { fn pop(&mut self) -> u16 {
let val = self.mem[self.tos]; let val = self.mem[self.tos];
self.mem[self.tos] = 0; self.mem[self.tos] = 0;
@ -152,12 +151,12 @@ struct Core {
* Instead we are going with the "byte-addressed memory" that is more * Instead we are going with the "byte-addressed memory" that is more
* conventional in today's computers. This choice is arbitrary. * conventional in today's computers. This choice is arbitrary.
*/ */
ip: u16, // instruction pointer ip: u16, /* instruction pointer */
dstack: Stack<16>, // data stack dstack: Stack<16>, /* data stack */
rstack: Stack<32> // return stack rstack: Stack<32> /* return stack */
} }
/* Function to initialize the cpu. /* Helper to initialize the cpu.
* There is probably a better idiom for this but I am bad at rust */ * There is probably a better idiom for this but I am bad at rust */
fn new_core() -> Core { fn new_core() -> Core {
let c = Core { let c = Core {
@ -200,7 +199,7 @@ fn new_core() -> Core {
*/ */
impl Core { impl Core {
/* Helper function - Read a number from the specified memory address. */ /* Helper to read a number from the specified memory address. */
fn load(&self, addr: u16) -> u16 { fn load(&self, addr: u16) -> u16 {
let a = addr as usize; let a = addr as usize;
/* We immediately run into trouble because we are using byte-addressed /* We immediately run into trouble because we are using byte-addressed
@ -241,7 +240,7 @@ impl Core {
/* The le in this function call stands for little-endian. */ /* The le in this function call stands for little-endian. */
} }
/* Helper function - Write a number to the specified memory address. */ /* Helper to write a number to the specified memory address. */
fn store(&mut self, addr: u16, val: u16) { fn store(&mut self, addr: u16, val: u16) {
let a = addr as usize; let a = addr as usize;
self.ram[a..=a+1].copy_from_slice(&val.to_le_bytes()); self.ram[a..=a+1].copy_from_slice(&val.to_le_bytes());
@ -383,14 +382,14 @@ impl Core {
/* Now that the instruction set is generally described /* Now that the instruction set is generally described
* let's look at the code that implements it */ * let's look at the code that implements it */
fn step(&mut self) { fn step(&mut self) {
// 1. Fetch the instruction. /* 1. Fetch the instruction.
// Also advance ip to point at the next instruction for next time. * Also advance ip to point at the next instruction for next time. */
let opcode = self.load(self.ip); let opcode = self.load(self.ip);
self.ip = self.ip.wrapping_add(2); self.ip = self.ip.wrapping_add(2);
// 2. Decode and execute the instruction /* 2. Decode and execute the instruction */
if (opcode >= 0xffe0) && (opcode & 1 == 0) { if (opcode >= 0xffe0) && (opcode & 1 == 0) {
// Data processing instruction /* Data processing instruction */
PRIMITIVES[((opcode - 0xffe0) >> 1) as usize](self); PRIMITIVES[((opcode - 0xffe0) >> 1) as usize](self);
/* These instructions get looked up in a table. The bit /* These instructions get looked up in a table. The bit
* math converts the instruction code into an index in the * math converts the instruction code into an index in the
@ -406,18 +405,18 @@ impl Core {
*/ */
} }
else if (opcode & 1) == 1 { else if (opcode & 1) == 1 {
// Literal /* Literal */
self.dstack.push(opcode >> 1); self.dstack.push(opcode >> 1);
} }
else { else {
// Call /* Call */
self.rstack.push(self.ip); self.rstack.push(self.ip);
self.ip = opcode; self.ip = opcode;
} }
} }
} }
// The names of the 16 remaining CPU instructions /* The names of the 16 remaining CPU instructions */
enum Op { enum Op {
RET = 0xffe0, TOR = 0xffe2, RTO = 0xffe4, LD = 0xffe6, RET = 0xffe0, TOR = 0xffe2, RTO = 0xffe4, LD = 0xffe6,
ST = 0xffe8, DUP = 0xffea, SWP = 0xffec, DRP = 0xffee, ST = 0xffe8, DUP = 0xffea, SWP = 0xffec, DRP = 0xffee,
@ -427,29 +426,29 @@ enum Op {
type Primitive = fn(&mut Core); type Primitive = fn(&mut Core);
// A table of functions for each of the 16 remaining CPU instructions /* A table of functions for each of the 16 remaining CPU instructions */
const PRIMITIVES: [Primitive; 16] = [ const PRIMITIVES: [Primitive; 16] = [
/* Return-stack instructions */ /* Return-stack instructions */
| x | { | x | {
// RET - Return from subroutine /* RET - Return from subroutine */
x.ip = x.rstack.pop() x.ip = x.rstack.pop()
}, },
| x | { | x | {
// TOR - Transfer number from data stack to return stack /* TOR - Transfer number from data stack to return stack */
x.rstack.push(x.dstack.pop()) x.rstack.push(x.dstack.pop())
}, },
| x | { | x | {
// RTO - Transfer number from return stack to data stack /* RTO - Transfer number from return stack to data stack */
x.dstack.push(x.rstack.pop()) x.dstack.push(x.rstack.pop())
}, },
/* Memory instructions */ /* Memory instructions */
| x | { | x | {
// LD - Load number from memory address specified on the data stack /* LD - Load number from memory address specified on the data stack */
let a = x.dstack.pop(); let a = x.dstack.pop();
x.dstack.push(x.load(a)); x.dstack.push(x.load(a));
}, },
| x | { | x | {
// ST - Store number to memory address specified on the data stack /* ST - Store number to memory address specified on the data stack */
let a = x.dstack.pop(); let a = x.dstack.pop();
let v = x.dstack.pop(); let v = x.dstack.pop();
x.store(a, v); x.store(a, v);
@ -607,43 +606,133 @@ const PRIMITIVES: [Primitive; 16] = [
* Part 2 - The Program * Part 2 - The Program
* ------------------------------------------------------------------------ */ * ------------------------------------------------------------------------ */
/* You now have an unfamiliar computer with no software. It sits there doing /* "In a sense we're building a tree. We've now reached a
* nothing. Can you and the computer write a program? * point where we can start making the roots. For a while
* everything will be concealed but we'll eventually reach
* daylight and start on branches."
* *
* We are going to need to give the computer a list of instructions, by * -- Chuck Moore, "Programming a Problem-Oriented Language", 1970
* which I mean a list of numbers. If we were sitting in front of a * https://colorforth.github.io/POL.htm
* minicomputer in 196x, we would need a copy of the target machine's
* instruction set, paper and pencil, and a lot of coffee.
* *
* In 20xx we are fortunate enough to have rust so we will put it to work. * You now have an unfamiliar computer with no software. Can you and the
* computer write a program?
* *
* Regardless, this bootstrapping process isn't going to be very pleasant * The first program is the hardest to write because you don't have any tools to
* so the goal is to make it short. We want a language that can stand on * help write it. The computer itself is going to be no help. Without any
* its own as quickly as possible, so the computer can start helping us * program it will sit there doing nothing.
* write the program. *
* What should the first program be?
* A natural choice would be a tool that helps you program more easily.
*
* An interactive programming environment needs to let you do 2 things:
*
* 1. Call subroutines by typing their name at the keyboard
* 2. Define new subroutines in terms of existing ones
*
* Begin with step 1:
* Call subroutines by typing their name at the keyboard
*
* This is where we will meet Forth.
*
* The below is a small Forth for bootstrapping this computer. If you want to
* learn how to implement a full featured Forth, please read Jonesforth, and
* Brad Rodriguez' series of articles "Moving Forth". The simple Forth I write
* below will probably help you understand those Forths a little better.
*
* Forth organizes all the computer's memory as a "dictionary" of subroutines.
* The point of the dictionary is to give each subroutine a name so you
* can run a subroutine by typing its name. The computer will look up its
* address for you and call it.
* *
* Forth is a weird language but its design decisions make a lot of sense
* if you view it as a bootstrapping tool.
*/
/*
* Forth organizes all the computer's memory as a "dictionary".
* The dictionary starts at a low address and grows towards high addresses. * The dictionary starts at a low address and grows towards high addresses.
* It is organized like a linked-list, like this: * It is organized like a linked-list, like this:
* *
* [Link field][Name][Code .......... ]
* ^
* |
* [Link field][Name][Code ...... ]
* ^
* |
* [Link field][Name][Code ............... ]
* *
* Code is stored in the dictionary as a list of addresses. * The reason it is a linked list is to allow each list entry to be a
* different length.
* *
* Each dictionary entry contains three things:
* *
* - "Link field": The address of the previous dictionary entry.
* For the first dictionary entry this field is 0.
*
* - "Name": A few letters to name this dictionary entry.
* Later you will type this name at the keyboard to call up
* this dictionary entry.
*
* - "Code": A subroutine to execute when you call up this dictionary
* entry. This is a list of CPU instructions. Note that one
* of the CPU instructions is "call". So you can have a subroutine
* that call other subroutines, or calls itself.
*
* This code must end with a return (RET) instruction.
*
* Example subroutine:
*
* Number Instruction Meaning
* ------ ----------- -------
* 7 Literal(3) Push the value 3 onto the data stack
* 9 Literal(4) Push the value 4 onto the data stack
* 65504 RET Return to caller
*
* A linked list is not a very fast data structure but this doesn't really
* matter because dictionary lookup doesn't need to be fast. Lookups are
* for converting text you typed at the keyboard to subroutine addresses.
* You can't type very fast compared to a computer so this lookup doesn't
* need to be fast.
*
* In addition to the linked list itself, you will need a couple of
* variables to keep track of where the dictionary is in memory:
*
* - Dictionary pointer: The address of the newest dictionary entry.
* - Here: The address of the first unused memory location,
* which comes just after the newest dictionary entry.
*
* [Link field][Name][Code .......... ]
* ^
* |
* [Link field][Name][Code ...... ]
* ^
* |
* [Link field][Name][Code ............... ]
* ^ ^
* | |
* [Dictionary pointer] [Here]
*
* Got all that?
*
* To create our Forth interactive programmming environment, we will start
* by defining subroutines that:
* - read names from the keyboard
* - look up and execute dictionary entries by name
*
* We will put these subroutines themselves in the dictionary so they are
* available for use once our interactive environment is up and running!
*
* If you were sitting in front of a microcomputer in 196x you would need
* to create the dictionary with pencil and paper, but in 20xx we will
* write a Rust program to help create the dictionary.
*
* First we need to keep track of where the dictionary is:
*/ */
/* Here is the stuff that you would normally be doing with pencil and paper */
struct Dict<'a> { struct Dict<'a> {
dp: u16, dp: u16, // The dictionary pointer
here: u16, here: u16, // The "here" variable
c: &'a mut Core c: &'a mut Core // The dictionary lives in memory. We are going to
// hang on to a mutable reference to the core to give
// us easy access to the memory.
} }
/* Helpers to help put new routines in the dictionary */
enum Item { enum Item {
Literal(u16), Literal(u16),
Call(u16), Call(u16),
@ -653,15 +742,19 @@ impl From<u16> for Item { fn from(a: u16) -> Self { Item::Call(a) } }
impl From<Op> for Item { fn from(o: Op) -> Self { Item::Opcode(o) } } impl From<Op> for Item { fn from(o: Op) -> Self { Item::Opcode(o) } }
impl Dict<'_> { impl Dict<'_> {
/* Helper to reserve space in the dictionary by advancing the "here"
* pointer */
fn allot(&mut self, n: u16) { fn allot(&mut self, n: u16) {
self.here = self.here.wrapping_add(n); self.here = self.here.wrapping_add(n);
} }
/* Helper to append a 16 bit integer to the dictionary */
fn comma(&mut self, val: u16) { fn comma(&mut self, val: u16) {
self.c.store(self.here, val); self.c.store(self.here, val);
self.allot(2); self.allot(2);
} }
/* Helper to append a CPU instruction to the dictionary */
fn emit<T: Into<Item>>(&mut self, val: T) { fn emit<T: Into<Item>>(&mut self, val: T) {
match val.into() { match val.into() {
Item::Call(val) => { self.comma(val) } Item::Call(val) => { self.comma(val) }
@ -671,11 +764,36 @@ impl Dict<'_> {
} }
} }
/* Helper to append a "name" field to the dictionary. To save space and
* to make each dictionary header a consistent size, I am choosing to not
* store every letter of the name. Instead I am storing only the length of
* the name and then the first three letters of the name.
*
* That means these two names will compare equal:
* - ALLOW (-> 5ALL)
* - ALLOT (-> 5ALL)
*
* Even though their first three letters are the same, these two names
* will compare unequal because they are different lengths:
* - FORTH (-> 5FOR)
* - FORGET (-> 6FOR)
*
* If a name is shorter than 3 letters it is padded out with spaces.
* - X (-> 1X )
*
* You can see that the name field is always four bytes regardless
* of how many letters are in the name, and the link field is two bytes.
* This means a dictionary header in this Forth is always six bytes.
*/
fn name(&mut self, n: u8, val: [u8; 3]) { fn name(&mut self, n: u8, val: [u8; 3]) {
/* Store the length and the first character */
self.comma(n as u16 | ((val[0] as u16) << 8)); self.comma(n as u16 | ((val[0] as u16) << 8));
/* Store the next two characters */
self.comma(val[1] as u16 | ((val[2] as u16) << 8)); self.comma(val[1] as u16 | ((val[2] as u16) << 8));
} }
/* Helper to append a new link field to the dictionary and update the
* dictionary pointer appropriately. */
fn entry(&mut self) { fn entry(&mut self) {
let here = self.here; let here = self.here;
self.comma(self.dp); self.comma(self.dp);
@ -683,44 +801,371 @@ impl Dict<'_> {
} }
} }
/* Now we can start building the dictionary. */
fn build_dictionary(c: &mut Core) { fn build_dictionary(c: &mut Core) {
use Op::*; use Op::*;
use Item::*; use Item::*;
let mut d = Dict {dp: 0, here: 2, c: c}; let mut d = Dict {
dp: 0, /* Nothing in the dictionary yet */
here: 2, /* Reserve address 0 as an "entry point", i.e. where the
CPU will jump to start running Forth. We don't have a
Forth interpreter yet so we'll leave address 0 alone for
now and start the dictionary at address 2 instead. */
c: c
};
/* Consider the following facts:
* - The CPU knows how to execute a bunch of instructions strung together.
* - Forth consists of a bunch of subroutine calls strung together.
* - Subroutine CALL is a valid instruction of our CPU.
*
* This means that we can immediately begin programming our machine in
* a language resembling Forth, just by writing a list of subroutine
* calls into the dictionary.
*
* The line between "machine code program" and "Forth program" is
* very blurry. To illustrate:
*
* Here is a subroutine consisting of a few instructions strung together.
*
* Instruction Number Meaning
* ----------- ------ -------
* Literal(3) 7 Push the value 3 onto the data stack
* Literal(4) 9 Push the value 4 onto the data stack
* RET 65504 Return to caller
*
* Here is a Forth subroutine consisting of a few subroutine calls strung
* together.
* Call Number Meaning
* ----------- ------ -------
* S1 1230 Call subroutine S1 which happens to live
* at address 1230
* S2 1250 Call subroutine S2 which happens to live
* at address 1250
* RET 65504 Return to caller
*
* This duality between CPU instructions and Forth code comes from
* an idea called "subroutine threading". It is a refinement of an
* idea called "threaded code". This has no relation to the kind of
* threading that lets you run programs in parallel. You can read more
* about threaded code on Wikipedia or in the other Forth resources I
* mentioned earlier (Jonesforth, and Moving Forth by Brad Rodriguez).
*
* Our new language starts out with the sixteen (well, eighteen)
* instructions built into the CPU. We can string those instructions
* together into a new subroutine. This subroutine adds to the pool
* of functions we have available for making new subroutines.
*
* Repeat until you have built what you wanted to build, via
* function composition. This is the idea behind Forth.
*/
/*
* We are going to be writing many series of instructions so let's
* start out by making a Rust macro that makes them easier to type
* and lets us specify a CPU instruction vs. a subroutine call with
* equal ease.
*
* The macro below will convert:
*
* forth!(Literal(2), ADD, RET)
*
* to:
*
* d.emit(Literal(2));
* d.emit(ADD);
* d.emit(RET);
*
* which you probably recognize as code that will add a new subroutine
* to the dictionary.
*/
macro_rules! forth { macro_rules! forth {
($x:expr) => (d.emit($x)); ($x:expr) => (d.emit($x));
($x:expr, $($y:expr),+) => (d.emit($x); forth!($($y),+)) ($x:expr, $($y:expr),+) => (d.emit($x); forth!($($y),+))
} }
/* Now we can add the first subroutine to the dictionary!
*
* key: Reads a character from the keyboard and places its character
* code on the stack.
*
* There is a tradition of writing stack comments for Forth subroutines
* to describe the stack effect of executing the subroutine.
* They look like this: key ( -- n )
*
* Read as: key does not take any parameters off the stack, and leaves
* one new number pushed onto the stack.
*
* Also remember that a dictionary entry looks like this:
* [Link field][Name][Code .......... ]
*/
// key ( -- n ) // key ( -- n )
d.entry(); d.name(3, *b"key"); let key = d.here; d.entry(); /* Compile the link field into the dictionary */
forth!(Literal(0), IO, RET); d.name(3, *b"key"); /* Compile the name field into the dictionary */
let key = d.here; /* (Save off the start address of the code so we
can call it later) */
forth!(
Literal(0), /* Compile a LITERAL instruction that pushes
0 to the stack */
IO, /* Compile an IO instruction.
*
* Remember from the CPU code that IO takes a
* parameter on the stack to specify which port
* to use.
*
* Also remember that IO port 0 reads
* a character from standard input.
*/
RET /* Compile a RET instruction */
);
/* We have now compiled the "key" subroutine into the dictionary.
* [Link field][Name][Code .......... ]
* 0000 3key 1, 65534, 65504
*
* The next subroutine we will make is "emit". This is a companion
* to "key" that works in the opposite direction.
*
* key ( -- n ) reads a character from stdin and pushes it to the stack.
* emit ( n -- ) pops a character from the stack and writes it to stdout.
*/
// emit ( n -- ) // emit ( n -- )
d.entry(); d.name(4, *b"emi"); let emit = d.here; d.entry(); d.name(4, *b"emi"); let emit = d.here;
forth!(Literal(1), IO, RET); forth!(Literal(1), IO, RET);
/* I am tired of saying "subroutine" so many times, so I am going to
* introduce a new term. Remember the goal our language is working
* towards -- we want to be able to type a word at the keyboard, and
* let the computer look it up in the dictionary and execute the
* appropriate code.
*
* So far we have two named items in the dictionary, call and emit.
*
* We are going to term a named dictionary item a "word".
* This is a Forth tradition.
*
* So call and emit are "words", or "dictionary words" if you want to be
* precise about it. So far these are the only words we've defined.
*
* Let's define some more words.
*/
/* Our CPU does not have subtraction so let's make subtraction by adding
* the two's complement.
*
* To get the two's complement, do a bitwise invert and add 1.
*
* This will be the most complicated Forth that we've written so far
* so let's walk through step by step. */
// - ( a b -- a-b ) // - ( a b -- a-b )
d.entry(); d.name(1, *b"- "); let sub = d.here; d.entry(); d.name(1, *b"- "); let sub = d.here;
forth!(INV, Literal(1), ADD, ADD, RET); forth!( /* Stack contents: a b, to start off with.
* We want to compute a minus b */
INV, /* Bitwise invert the top item on the stack.
* Stack contents: a ~b */
Literal(1), /* Push 1 onto the stack.
* Stack contents: a ~b 1 */
ADD, /* Add the top two items on the stack.
* Stack contents: a ~b+1
* Note that ~b+1 is the two's complement of b. */
ADD, /* Add the top two items on the stack.
* Stack contents: n
* Note that n = (a + ~b+1) = a - b */
RET /* Done, return to caller, leaving n on the data stack. */
);
/* Writing it out like that takes a lot of space. Normally Forth code
* is written on a single line, like this:
*
* INV 1 ADD ADD RET
*
* Looking at it this way, it's easy to see the new word we just
* created (-) is made from 5 instructions. It's pretty typical for
* a Forth word to be made of 2-7 of them. Beyond that length, things
* get successively harder to understand, and it becomes a good idea
* to split some work off into helper words.
*
* We will see an example of this below.
*/
/* Our next word will be useful for Boolean logic.
*
* 0= ( n -- f )
*
* In a stack comment, "f" means "flag", a.k.a. Boolean value.
* By Forth convention, zero is false and any nonzero value is true.
* However the "best" value to use for a true flag is 65535 (all ones)
* so the bitwise logical operations can double as Boolean logical
* operations.
*
* So what 0= does is:
* - if n=0, leave on the stack f=65535
* - otherwise, leave on the stack f=0
*
* It is like C's ! operator.
*
* In Rust this could be implemented as:
*
* fn zero_eq(n: u16) {
* if (n == 0) {
* return 65535;
* } else {
* return 0;
* }
* }
*
* Rust has an if-then and block scope, so this is easy to write.
*
* The literal translation to a typical register-machine assembly
* language would look something like this:
*
* zero_eq: compare r0, 0
* jump_eq is_zero
* move r0, 0
* ret
* is_zero: move r0, 65535
* ret
*
* It looks simple but I want to point out a couple things about it
* that are not so simple.
*
* The conditional jump instruction, jump_eq.
* ------------------------------------------
* Our CPU doesn't have this. Q is the only "decision-making"
* instruction that our CPU has.
*
* Q - If the top number on the data stack is zero, skip the next
* instruction.
*
* The forward reference
* ---------------------
* This is another problem. Think of the job of an assembler which is
* converting an assembly language program to machine code. We are
* currently writing our code in a tiny assembler that we made in Rust! It
* is very simple but so far it has worked for us. The assembler of our
* hypothetical register-machine below has a rather nasty problem to solve.
*
* zero_eq: compare r0, 0
* jump_eq is_zero <----- On this line.
* move r0, 0
* ret
* is_zero: move r0, 65535
* ret
*
* It wants to jump to is_zero but that symbol has not been seen yet and is
* unrecognized. On top of that, the assembler also doesn't yet know what
* address is_zero will have, so doesn't know what jump target to emit.
* To successfully assemble that kind of program you would need an
* assembler smarter than the assembler we made for ourselves in Rust.
*
* There are ways to solve this but let's NOT solve it.
*
* Our CPU has no jump instruction (only call) and our assembler only lets
* us call things we already defined. Instead of removing these
* constraints, find a way to write 0= within the constraints.
*
* Here is a start at solving the problem
*
* is_nonzero ( -- 0 )
* Literal(0)
* RET
*
* 0= ( n -- f )
* Q <-- pop n, if n=0 skip next instruction
* is_nonzero <-- f=0 is now pushed to stack
* Literal(0)
* INV <-- f=65535 is now pushed to stack
* RET <-- Return
*
* We got rid of the forward reference by defining is_nonzero before it
* was used.
*
* We got rid of the jump instruction by using a subroutine call instead.
*
* This code is close to working but it doesn't quite work. The problem
* is that is_nonzero gives control back to 0= when done, just like
* a subroutine call normally does, and then 0= runs as normal until it
* hits the return instruction at the end.
* So we wind up executing both the f=0 branch and the f=65535 branch,
* instead of just executing the f=0 branch like we wanted in this case.
*
* It is possible to fix this last problem by adding the instructions
* RTO DRP to is_nonzero.
*
* is_nonzero ( -- 0 )
* RTO <-- Pop the return address, push to data stack
* DRP <-- Discard it
* Literal(0) <-- Put 0 on the data stack
* RET <-- Return
*
* Because we popped off and discarded one item from the return stack, the
* final RET instruction will not return to 0= any more. Instead it will
* skip one level and return to whoever called 0=. This has the result of
* ending 0= early, which is what we wanted to do.
*
* I call this pattern "return-from-caller". It is used occasionally in
* real Forth systems. My dialect of Forth will use it extensively to work
* around my CPU's lack of conditional branch.
*
* Now we've explained how 0= is going to work, let's make it.
*/
/* First we define the helper. It won't be reused, so I am not going
* to bother giving it a dictionary header and name for easy lookup later.
* Think of it as a private function. */
let zero = d.here; let zero = d.here;
forth!(Literal(0), RTO, DRP, RET); forth!(Literal(0), RTO, DRP, RET);
/* Now define 0= using the helper. */
// 0= ( n -- f ) // 0= ( n -- f )
d.entry(); d.name(2, *b"0= "); let zero_eq = d.here; d.entry(); d.name(2, *b"0= "); let zero_eq = d.here;
forth!(Q, zero, Literal(0), INV, RET); forth!(Q, zero, Literal(0), INV, RET);
/* Next let's make a = equality comparison operator, using 0= and subtract.
* I call it an "operator" because that's what other languages would
* call it, but Forth has no special idea of an "operator". Everything
* is just words. */
// = ( a b -- a=b ) // = ( a b -- a=b )
d.entry(); d.name(1, *b"= "); let eq = d.here; d.entry(); d.name(1, *b"= "); let eq = d.here;
forth!(sub, zero_eq, RET); forth!(sub, zero_eq, RET);
/* Note that 0= and subtract are both words, not CPU instructions.
* This makes = the first "pure" Forth word we have defined, with no
* direct dependency on the machine's instruction set.
* We could define = as - 0= on a real standards-compliant Forth system
* and it would still work. So Forth gets you to the point of writing
* "portable" code really quickly. Often you can reuse routines early in
* bootstrapping even though they were written and tested on a different
* machine. Many languages offer portability but few offer it so quickly.
*/
// Advance past whitespace /* Now that we've got some basics in place let's go back to solving
* the real problem of getting our language to read words from the
* keyboard. The first problem we have is that we need some way to
* separate words from each other so we know where one word ends and the
* next begins. This problem is called "lexing". Forth has about the
* simplest lexer ever, it just splits on whitespace. Anything with
* character code <=32 is considered whitespace. Words are delimited by
* whitespace. And that is all the syntax Forth has.
*
* To read a word from the keyboard you will need to:
* - Advance past any leading whitespace
* - Read characters into a buffer until whitespace is seen again.
*/
/* Let's start with the "advance past whitespace" part */
let skip_helper = d.here; let skip_helper = d.here;
forth!(RTO, DRP, key, DUP, Literal(33), GEQ, Q, RET, DRP, skip_helper); forth!(RTO, DRP, key, DUP, Literal(33), GEQ, Q, RET, DRP, skip_helper);
// skipws ( -- c )
d.entry(); d.name(6, *b"ski"); let skipws = d.here; d.entry(); d.name(6, *b"ski"); let skipws = d.here;
forth!(skip_helper); forth!(skip_helper);
@ -748,13 +1193,13 @@ fn build_dictionary(c: &mut Core) {
d.entry(); d.name(2, *b"c! "); let cst = d.here; d.entry(); d.name(2, *b"c! "); let cst = d.here;
forth!(DUP, LD, Literal(0xff), INV, AND, SWP, TOR, OR, RTO, ST, RET); forth!(DUP, LD, Literal(0xff), INV, AND, SWP, TOR, OR, RTO, ST, RET);
// Load 1 letter into buffer. /* Load 1 letter into buffer. */
let stchar = d.here; let stchar = d.here;
forth!(Literal(word_buf), cld, Literal(1), ADD, DUP, Literal(word_buf), cst, forth!(Literal(word_buf), cld, Literal(1), ADD, DUP, Literal(word_buf), cst,
Literal(5), min, Literal(word_buf), ADD, cst, RET); Literal(5), min, Literal(word_buf), ADD, cst, RET);
// Load letters into buffer until whitespace is hit again. /* Load letters into buffer until whitespace is hit again.
// Return the whitespace character that was seen. * Return the whitespace character that was seen. */
let getcs_helper = d.here; let getcs_helper = d.here;
forth!(RTO, DRP, stchar, key, DUP, Literal(32), SWP, GEQ, Q, RET, getcs_helper); forth!(RTO, DRP, stchar, key, DUP, Literal(32), SWP, GEQ, Q, RET, getcs_helper);
@ -769,8 +1214,8 @@ fn build_dictionary(c: &mut Core) {
skipws, getcs, DRP, RET); skipws, getcs, DRP, RET);
// latest ( -- a ) // latest ( -- a )
// Address of "latest" variable. This variable stores the address of /* Address of "latest" variable. This variable stores the address of
// the latest word in the dictionary. the latest word in the dictionary. */
let latest_ptr = d.here; d.allot(2); let latest_ptr = d.here; d.allot(2);
d.entry(); d.name(6, *b"lat"); let latest = d.here; d.entry(); d.name(6, *b"lat"); let latest = d.here;
forth!(Literal(latest_ptr), RET); forth!(Literal(latest_ptr), RET);
@ -806,15 +1251,15 @@ fn build_dictionary(c: &mut Core) {
forth!(DUP, DUP, Literal(3), SFT, ADD, ADD, RET); forth!(DUP, DUP, Literal(3), SFT, ADD, ADD, RET);
// here ( -- a ) // here ( -- a )
// Address of "here" variable. This variable stores the address of /* Address of "here" variable. This variable stores the address of
// the first free space in the dictionary the first free space in the dictionary */
let here_ptr = d.here; d.allot(2); let here_ptr = d.here; d.allot(2);
d.entry(); d.name(4, *b"her"); let here = d.here; d.entry(); d.name(4, *b"her"); let here = d.here;
forth!(Literal(here_ptr), RET); forth!(Literal(here_ptr), RET);
// state ( -- a ) // state ( -- a )
// Address of "state" variable. This variable stores -1 if /* Address of "state" variable. This variable stores -1 if
// interpreting or 0 if compiling. * interpreting or 0 if compiling. */
let state_ptr = d.here; d.allot(2); let state_ptr = d.here; d.allot(2);
d.entry(); d.name(5, *b"sta"); let state = d.here; d.entry(); d.name(5, *b"sta"); let state = d.here;
forth!(Literal(state_ptr), RET); forth!(Literal(state_ptr), RET);
@ -906,7 +1351,8 @@ fn build_dictionary(c: &mut Core) {
d.entry(); d.name(1 | 0x80, *b"; "); d.entry(); d.name(1 | 0x80, *b"; ");
forth!(Literal(!(RET as u16)), INV, comma, lbracket, unsmudge, RET); forth!(Literal(!(RET as u16)), INV, comma, lbracket, unsmudge, RET);
// Finally put the primitives in the dictionary so they can be called directly. /* Finally put the primitives in the dictionary so they can be
* called interactively. */
d.entry(); d.name(3, *b"ret"); forth!(RTO, DRP, RET); d.entry(); d.name(3, *b"ret"); forth!(RTO, DRP, RET);
d.entry(); d.name(2, *b">r "); forth!(RTO, SWP, TOR, TOR, RET); d.entry(); d.name(2, *b">r "); forth!(RTO, SWP, TOR, TOR, RET);
d.entry(); d.name(2, *b"r> "); forth!(RTO, RTO, SWP, TOR, RET); d.entry(); d.name(2, *b"r> "); forth!(RTO, RTO, SWP, TOR, RET);