3 You can run JONASFORTH inside QEMU or on real hardware. If you want to run
4 inside QEMU, you should have the following dependencies installed (assuming
9 Then, to run a UEFI shell inside QEMU, run:
13 JONASFORTH will be available as `main` on `FS0:`. Thus, to run it, you can run
14 the following command inside the UEFI shell:
18 S" Hello, World!" TELL
21 (Try typing in the code in `example.f` for something a little more
24 ## Running on real hardware
26 Making the program run on physical hardware is pretty easy. Just create a
27 FAT32-formatted USB drive, and copy `out/main` to it. Then, you can execute the
28 program in the same way that you did from inside QEMU, assuming your system
29 comes with a UEFI shell built-in.
31 If your system doesn't have a UEFI shell, then you can copy the executable to
32 `\EFI\BOOT\BOOTx64.EFI` on the USB drive. Then, the system should be able to
33 boot from the USB drive and directly into JONASFORTH. The way to do this is a
34 little bit different depending on the exact firmware, but most firmwares will
35 have some way to enter a boot menu where you can select the USB drive. You may
36 need to disable Secure Boot first.
38 To format a USB drive as FAT32, you can run
40 # mkfs.vfat -F32 /dev/sdx
42 with `/dev/sdx` replaced by the path of your USB drive. Then mount the drive,
43 and copy `out/main` to `\EFI\BOOT\BOOTx64.EFI`:
47 $ mkdir -p mnt/EFI/BOOT
49 $ cp out/main mnt/EFI/BOOT/BOOTx64.EFI
51 Now you should be able to boot directly from the USB drive.
53 # Notes on implementation
55 The implementation is based on
56 [JONESFORTH](https://raw.githubusercontent.com/nornagon/jonesforth/master/jonesforth.S).
57 This is my summary of the most important parts.
61 In Forth, words are stored in a dictionary. The dictionary is a linked list
62 whose entries look like this:
64 +------------------------+--------+---------- - - - - +----------- - - - -
65 | LINK POINTER | LENGTH/| NAME | DEFINITION
67 +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - -
69 For example, DOUBLE and QUADRUPLE may be stored like this:
71 pointer to previous word
74 +--|------+---+---+---+---+---+---+---+---+------------- - - - -
75 | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...)
76 +---------+---+---+---+---+---+---+---+---+------------- - - - -
79 +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
80 | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...)
81 +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
87 The Forth variable LATEST contains a pointer to the most recently defined word.
91 In a typical Forth interpreter, code is stored in a peculiar way. (This way of
92 storing code is primarily motivated by space contraints on early systems.)
94 The definition of a word is stored as a sequence of memory adresses of each of
95 the words making up that definition. (At the end of a compiled definition, there
96 is also some extra code that causes execution to continue in the correct way.)
98 We use a register (ESI) to store a reference to the next index of the
99 word (inside a definition) that we are executing. Then, in order to execute a
100 word, we just jump to whatever address is pointed to by ESI. The code for
101 updating ESI and continuing execution is stored at the end of each subroutine.
103 Of course, this approach only works if each of the words that we are executing
104 is defined in assembly, but we also want to be able to execute Forth words!
106 We get around this problem by adding a "codeword" to the beginning of any
107 compiled subroutine. This codeword is a pointer to the intrepreter to run the
108 given function. In order to run such functions, we actually need two jumps when
109 executing: In order to execute a word, we jump to the address at the location
110 pointed to by the address in ESI.
114 What does the codeword of a Forth word contain? It needs to save the old value
115 of ESI (so that we can resume execution of whatever outer definition we are
116 executing at the time) and set the new version of ESI to point to the first word
117 in the inner definition.
119 The stack where the values of ESI are stored is called the "return stack". We
120 will use EBP for the return stack.
122 As mentioned, whenever we finish executing a Forth word, we will need to
123 continue execution in the manner described in the previous section. When the
124 word being executed is itself written in Forth, we need to pop the old value of
125 ESI that we saved at the beginning of the definition before doing this.
127 Thus, the actual data for a word in a dictionary will look something like this:
129 pointer to previous word
132 +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
133 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
134 +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
137 LINK in next word points to codeword of DUP
139 Here, DOCOL (the codeword) is address of the simple interpreter described above,
140 while EXIT a word (implemented in assembly) that takes care of popping ESI and
141 continuing execution. Note that DOCOL, DUP, + and EXIT are all stored as
142 addresses which point to codewords.
146 Literals are handled in a special way. There is a word in Forth, called LIT,
147 implemented in assembly. When executed, this word looks at the next Forth
148 instruction (i.e. the value of ESI), and places that on the stack as a literal,
149 and then manipulates ESI to skip over the literal value.
151 ## Built-in variables
153 - **STATE** -- Is the interpreter executing code (0) or compiling a word (non-zero)?
154 - **LATEST** -- Points to the latest (most recently defined) word in the dictionary.
155 - **HERE** -- Points to the next free byte of memory. When compiling, compiled words go here.
156 - **S0** -- Stores the address of the top of the parameter stack.
157 - **BASE** -- The current base for printing and reading numbers.
161 `WORD` reads a word from standard input and pushes a string (in the form of an
162 address followed by the length of the string) to the stack. (It uses an internal
163 buffer that is overwritten each time it is called.)
165 `FIND` takes a word as parsed by `WORD` and looks it up in the dictionary. It
166 returns the address of the dictionary header of that word if it is found.
167 Otherwise, it returns 0.
169 `>CFA` turns a dictionary pointer into a codeword pointer. This is used when
174 The Forth word INTERPRET runs in a loop, reading in words (with WORD), looking
175 them up (with FIND), turning them into codeword pointers (with >CFA) and then
176 deciding what to do with them.
178 In immediate mode (when STATE is zero), the word is simply executed immediately.
180 In compilation mode, INTERPRET appends the codeword pointer to user memory
181 (which is at HERE). However, if a word has the immediate flag set, then it is
182 run immediately, even in compile mode.
184 ### Definition of `:` and `;`
186 The word `:` starts by reading in the new word. Then it creates a new entry for
187 that word in the dictoinary, updating the contents of `LATEST`, to which it
188 appends the word `DOCOL`. Then, it switches to compile mode.
190 The word `;` simply appends `EXIT` to the currently compiling definition and
191 then switches back to immediate mode.
193 These words rely on `,` to append words to the currently compiling definition.
194 This word simply appends some literal value to `HERE` and moves the `HERE`
199 `JONASFORTH` is runs without an operating system, instead using the facilities
200 provided by UEFI by running as a UEFI application. (Or rather, in the future it
201 hopefully will. Right now, it uses Linux.) This section contains some notes
202 about how this functionality is implemented.
204 I also wrote an entire tutorial that descirbes how to write and compile a
205 "Hello, World!" UEFI application, including how to run it on real hardware,
206 which you can find here: [Getting started with bare-metal
207 assembly](https://johv.dk/blog/bare-metal-assembly-tutorial.html).
209 ## Packaging and testing the image
211 UEFI expects a UEFI application to be stored in a FAT32 file system on a
212 GPT-partitioned disk.
214 Luckily, QEMU has a convenient way of making a subdirectory availabe as a
215 FAT-formatted disk (see [the relevant section in the QEMU User
216 Documentation](https://qemu.weilnetz.de/doc/qemu-doc.html#disk_005fimages_005ffat_005fimages)
217 for more information):
219 $ qemu-sytem-x86_64 ... -hda fat:/some/directory
221 We use this to easily test the image in QEMU; see the Makefile for more
222 information, or just run the `qemu` target to run the program inside of QEMU
223 (of course, you must have QEMU installed for this to work):
227 ## Interfacing with UEFI
229 From [OSDev Wiki](https://wiki.osdev.org/UEFI#How_to_use_UEFI):
231 > Traditional operating systems like Windows and Linux have an existing software
232 > architecture and a large code base to perform system configuration and device
233 > discovery. With their sophisticated layers of abstraction they don't directly
234 > benefit from UEFI. As a result, their UEFI bootloaders do little but prepare
235 > the environment for them to run.
237 > An independent developer may find more value in using UEFI to write
238 > feature-full UEFI applications, rather than viewing UEFI as a temporary
239 > start-up environment to be jettisoned during the boot process. Unlike legacy
240 > bootloaders, which typically interact with BIOS only enough to bring up the OS,
241 > a UEFI application can implement sophisticated behavior with the help of UEFI.
242 > In other words, an independent developer shouldn't be in a rush to leave
245 For `JONASFORTH`, I have decided to run as a UEFI application, taking advantage
246 of UEFI's features, including its text I/O features and general graphical device
247 drivers. Eventually, we would like to add some basic graphical drawing
248 capabilities to `JONASFORTH`, and it's my impression that this would be possible
249 using what is provided to us by UEFI.
251 A UEFI images is basically a windows EXE without symbol tables. There are three
252 types of UEFI images; we use the EFI application, which has subsystem `10`. It
253 is an x68-64 image, which has value `0x8664`.
255 UEFI applications use [Microsoft's 64-bit calling
256 convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention)
257 for x68-64 functions. See the linked article for a full description. Here is
260 - Integer or pointer arguments are given in RCX, RDX, R8 and R9.
261 - Additional arguments are pushed onto the stack from right to left.
262 - Integer or pointer values are returned in RAX.
263 - An integer-sized struct is passed directly; non-integer-sized structs are passed as pointers.
264 - The caller must allocate 32 bytes of "shadow space" on the stack immediately
265 before calling the function, regardless of the number of parameters used, and
266 the caller is responsible for popping the stack afterwards.
267 - The following registers are volatile (caller-saved): RAX, RCX, RDX, R8, R9, R10, R11
268 - The following registers are nonvolatile (callee-saved): RBX, RBP, RDI, RSI, RSP, R12, R13, R14, R15
270 When the application is loaded, RCX contains a firmware allocated `EFI_HANDLE`
271 for the UEFI image, RDX contains a `EFI_SYSTEM_TABLE*` pointer to the EFI system
272 table and RSP contains the return address. For more infromation about how a UEFI
273 application is entered, see "4 - EFI System Table" in [the latest UEFI
274 specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf).
278 - [UEFI applications in detail - OSDev Wiki](https://wiki.osdev.org/UEFI#UEFI_applications_in_detail)
279 - [Microsoft x64 calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention)
280 - [UEFI Specifications](https://uefi.org/specifications)
284 We might want to consider using something like this: https://wiki.osdev.org/Uefi.inc)
286 FASM can generate UEFI application binaries by default. Use the following
287 template to output a 64-bit UEFI application:
292 section '.text' code executable readable
298 section '.data' data readable writable
302 Use `objdump -x` to inspect the assembled application binary.
304 ### UEFI documentation
306 - [Latest specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf)
311 - 4\. EFI System Table (89)
312 - 7\. Services - Boot Services (140)
313 - 8\. Services - Runtime Services (228)
314 - 12\. Protocols - Console Support (429)
315 - 13\. Protocols - Media Access (493)
316 - Appendix B - Console (2201)
317 - Appendix D - Status Codes (2211)
321 - [UEFI - OSDev Wiki](https://wiki.osdev.org/UEFI)
322 - [Unified Extensible Firmware Interface (Wikipedia)](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface)
323 - [UEFI Specifications](https://uefi.org/specifications)