7 The `sys.f` file contains code that defines some of the usual words that you
8 would expect in a Forth distribution. To run this code and then read from
11 $ cat sys.f - | ./main
13 The `example.f` file contains an example that you can run with:
15 $ cat sys.f example.f | ./main
19 We are currently in the process of implementing support for running without
20 Linux, by instead relying on UEFI. Eventually, this will be the only supported
21 method of running the interpreter, but currently the UEFI-related code is
22 isolated in the `uefi/` directory and does not yet contain an implementation of
25 You should have the following dependencies installed (assuming Arch Linux):
29 To run a UEFI shell inside qemu, cd to `uefi/` and run:
33 ### Running on read hardware
35 * [ ] This is not supported yet
37 # Notes on implementation
39 This is my summary of the most important parts of
40 https://raw.githubusercontent.com/nornagon/jonesforth/master/jonesforth.S.
44 In Forth, words are stored in a dictionary. The dictionary is a linked list
45 whose entries look like this:
47 +------------------------+--------+---------- - - - - +----------- - - - -
48 | LINK POINTER | LENGTH/| NAME | DEFINITION
50 +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - -
52 For example, DOUBLE and QUADRUPLE may be stored like this:
54 pointer to previous word
57 +--|------+---+---+---+---+---+---+---+---+------------- - - - -
58 | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...)
59 +---------+---+---+---+---+---+---+---+---+------------- - - - -
62 +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
63 | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...)
64 +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
70 The Forth variable LATEST contains a pointer to the most recently defined word.
74 In a typical Forth interpreter, code is stored in a peculiar way. (This way of
75 storing code is primarily motivated by space contraints on early systems.)
77 The definition of a word is stored as a sequence of memory adresses of each of
78 the words making up that definition. (At the end of a compiled definition, there
79 is also some extra code that causes execution to continue in the correct way.)
81 We use a register (ESI) to store a reference to the next index of the
82 word (inside a definition) that we are executing. Then, in order to execute a
83 word, we just jump to whatever address is pointed to by ESI. The code for
84 updating ESI and continuing execution is stored at the end of each subroutine.
86 Of course, this approach only works if each of the words that we are executing
87 is defined in assembly, but we also want to be able to execute Forth words!
89 We get around this problem by adding a "codeword" to the beginning of any
90 compiled subroutine. This codeword is a pointer to the intrepreter to run the
91 given function. In order to run such functions, we actually need two jumps when
92 executing: In order to execute a word, we jump to the address at the location
93 pointed to by the address in ESI.
97 What does the codeword of a Forth word contain? It needs to save the old value
98 of ESI (so that we can resume execution of whatever outer definition we are
99 executing at the time) and set the new version of ESI to point to the first word
100 in the inner definition.
102 The stack where the values of ESI are stored is called the "return stack". We
103 will use EBP for the return stack.
105 As mentioned, whenever we finish executing a Forth word, we will need to
106 continue execution in the manner described in the previous section. When the
107 word being executed is itself written in Forth, we need to pop the old value of
108 ESI that we saved at the beginning of the definition before doing this.
110 Thus, the actual data for a word in a dictionary will look something like this:
112 pointer to previous word
115 +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
116 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
117 +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
120 LINK in next word points to codeword of DUP
122 Here, DOCOL (the codeword) is address of the simple interpreter described above,
123 while EXIT a word (implemented in assembly) that takes care of popping ESI and
124 continuing execution. Note that DOCOL, DUP, + and EXIT are all stored as
125 addresses which point to codewords.
129 Literals are handled in a special way. There is a word in Forth, called LIT,
130 implemented in assembly. When executed, this word looks at the next Forth
131 instruction (i.e. the value of ESI), and places that on the stack as a literal,
132 and then manipulates ESI to skip over the literal value.
134 ## Built-in variables
136 * **STATE** -- Is the interpreter executing code (0) or compiling a word (non-zero)?
137 * **LATEST** -- Points to the latest (most recently defined) word in the dictionary.
138 * **HERE** -- Points to the next free byte of memory. When compiling, compiled words go here.
139 * **S0** -- Stores the address of the top of the parameter stack.
140 * **BASE** -- The current base for printing and reading numbers.
144 `WORD` reads a word from standard input and pushes a string (in the form of an
145 address followed by the length of the string) to the stack. (It uses an internal
146 buffer that is overwritten each time it is called.)
148 `FIND` takes a word as parsed by `WORD` and looks it up in the dictionary. It
149 returns the address of the dictionary header of that word if it is found.
150 Otherwise, it returns 0.
152 `>CFA` turns a dictionary pointer into a codeword pointer. This is used when
157 The Forth word INTERPRET runs in a loop, reading in words (with WORD), looking
158 them up (with FIND), turning them into codeword pointers (with >CFA) and then
159 deciding what to do with them.
161 In immediate mode (when STATE is zero), the word is simply executed immediately.
163 In compilation mode, INTERPRET appends the codeword pointer to user memory
164 (which is at HERE). However, if a word has the immediate flag set, then it is
165 run immediately, even in compile mode.
167 ### Definition of `:` and `;`
169 The word `:` starts by reading in the new word. Then it creates a new entry for
170 that word in the dictoinary, updating the contents of `LATEST`, to which it
171 appends the word `DOCOL`. Then, it switches to compile mode.
173 The word `;` simply appends `EXIT` to the currently compiling definition and
174 then switches back to immediate mode.
176 These words rely on `,` to append words to the currently compiling definition.
177 This word simply appends some literal value to `HERE` and moves the `HERE`
182 `JONASFORTH` is runs without an operating system, instead using the facilities
183 provided by UEFI by running as a UEFI application. (Or rather, in the future it
184 hopefully will. Right now, it uses Linux.) This section contains some notes
185 about how this functionality is implemented.
187 ## Packaging and testing the image
189 UEFI expects a UEFI application to be stored in a FAT32 file system on a
190 GPT-partitioned disk.
192 Luckily, QEMU has a convenient way of making a subdirectory availabe as a
193 FAT-formatted disk (see [the relevant section in the QEMU User
194 Documentation](https://qemu.weilnetz.de/doc/qemu-doc.html#disk_005fimages_005ffat_005fimages)
195 for more information):
197 $ qemu-sytem-x86_64 ... -hda fat:/some/directory
199 We use this to easily test the image in QEMU; see the Makefile for more information.
201 * [ ] How to build the image for real hardware (what should the image look like,
202 which programs, commands, etc.)
204 ## Interfacing with UEFI
206 From [OSDev Wiki](https://wiki.osdev.org/UEFI#How_to_use_UEFI):
208 >Traditional operating systems like Windows and Linux have an existing software
209 >architecture and a large code base to perform system configuration and device
210 >discovery. With their sophisticated layers of abstraction they don't directly
211 >benefit from UEFI. As a result, their UEFI bootloaders do little but prepare
212 >the environment for them to run.
214 >An independent developer may find more value in using UEFI to write
215 >feature-full UEFI applications, rather than viewing UEFI as a temporary
216 >start-up environment to be jettisoned during the boot process. Unlike legacy
217 >bootloaders, which typically interact with BIOS only enough to bring up the OS,
218 >a UEFI application can implement sophisticated behavior with the help of UEFI.
219 >In other words, an independent developer shouldn't be in a rush to leave
222 For `JONASFORTH`, I have decided to run as a UEFI application, taking advantage
223 of UEFI's features, including its text I/O features and general graphical device
224 drivers. Eventually, we would like to add some basic graphical drawing
225 capabilities to `JONASFORTH`, and it's my impression that this would be possible
226 using what is provided to us by UEFI.
228 A UEFI images is basically a windows EXE without symbol tables. There are three
229 types of UEFI images; we use the EFI application, which has subsystem `10`. It
230 is an x68-64 image, which has value `0x8664`.
232 UEFI applications use [Microsoft's 64-bit calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention) for x68-64 functions. See the linked article for a full description. Here is the short version:
234 * Integer or pointer arguments are given in RCX, RDX, R8 and R9.
235 * Additional arguments are pushed onto the stack from right to left.
236 * Integer or pointer values are returned in RAX.
237 * An integer-sized struct is passed directly; non-integer-sized structs are passed as pointers.
238 * The caller must allocate 32 bytes of "shadow space" on the stack immediately
239 before calling the function, regardless of the number of parameters used, and
240 the caller is responsible for popping the stack afterwards.
241 * The following registers are volatile (caller-saved): RAX, RCX, RDX, R8, R9, R10, R11
242 * The following registers are nonvolatile (callee-saved): RBX, RBP, RDI, RSI, RSP, R12, R13, R14, R15
244 When the application is loaded, RCX contains a firmware allocated `EFI_HANDLE`
245 for the UEFI image, RDX contains a `EFI_SYSTEM_TABLE*` pointer to the EFI system
246 table and RSP contains the return address. For more infromation about how a UEFI
247 application is entered, see "4 - EFI System Table" in [the latest UEFI
248 specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf).
252 * [UEFI applications in detail - OSDev Wiki](https://wiki.osdev.org/UEFI#UEFI_applications_in_detail)
253 * [Microsoft x64 calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention)
254 * [UEFI Specifications](https://uefi.org/specifications)
258 We might want to consider using something like this: https://wiki.osdev.org/Uefi.inc)
260 FASM can generate UEFI application binaries by default. Use the following
261 template to output a 64-bit UEFI application:
266 section '.text' code executable readable
272 section '.data' data readable writable
276 Use `objdump -x` to inspect the assembled application binary.
278 ### UEFI documentation
280 * [Latest specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf)
285 * 4\. EFI System Table (89)
286 * 7\. Services - Boot Services (140)
287 * 8\. Services - Runtime Services (228)
288 * 12\. Protocols - Console Support (429)
289 * 13\. Protocols - Media Access (493)
290 * Appendix B - Console (2201)
291 * Appendix D - Status Codes (2211)
295 * [UEFI - OSDev Wiki](https://wiki.osdev.org/UEFI)
296 * [Unified Extensible Firmware Interface (Wikipedia)](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface)
297 * [UEFI Specifications](https://uefi.org/specifications)