|Ondřej Hruška 169c003980 fix tests and warnings||6 months ago|
|_stash||7 months ago|
|crsn||6 months ago|
|crsn_arith||6 months ago|
|crsn_buf||6 months ago|
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|crsn_stdio||6 months ago|
|examples||6 months ago|
|launcher||6 months ago|
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|Cargo.toml||6 months ago|
|README.md||6 months ago|
|build.sh||7 months ago|
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|test_examples.sh||7 months ago|
Croissant (or crsn for short) is an extensible runtime emulating a weird microcomputer (or not so micro, that depends on what extensions you install).
F U N
Silly fast, actually. 60fps animations are perfectly doable if that's your thing. It's probably faster than you need for most things, actually.
You can slow it down using the
-C argument, or using sleep instructions.
Maybe this is not for you
Yes! You can use crsn as a scripting language!
The first line from a source file is skipped if it starts with
Yup, go ahead. You can also develop your own private crsn extensions, they work like plugins.
The runtime is built as a register machine with a stack and status flags.
Global registers are accessible everywhere. Other registers are only valid within an execution frame (in a routine, or the initial scope).
All registers are 64-bit unsigned integers that can be treated as signed, if you want to. Overflow is allowed and reported by status flags.
8-, 16-, 32-bit and floating point arithmetic is not currently implemented, but will be added later. Probably. Maybe.
Arithmetic and other operations set status flags that can be used for conditional jumps.
These keywords (among others) are used in conditional branches to specify flag tests:
nfull… Not full
nempty… Not empty
neof… Not EOF
else… Always true, may be used in the last branch
The syntax is very much subject to change at the moment. The format described here is valid at the time this file is added to version control.
Instructions are written using S-expressions, because they are easy to parse and everyone loves Lisp.
A program has this format:
( ...<instructions and routines>... )
( (ld r0 100) ; load value into a register (:again) ; a label (sub r0 1 ; subtract from a register (nz? ; conditional branch "not zero?" (j :again))) ; jump to the label :again )
The same program can be written in a compact form:
((ld r0 100)(:again)(sub r0 1 (nz? (j :again))))
Croissant allows program composition using includes.
Path can be a filesystem absolute or relative path (with slashes). Relative path is always resolved from the current source file. Use double quotes if needed.
If the included file is missing an extension,
.csn is appended automatically.
Each included file must contain a list of instructions or procedures, just like the main file.
Includes work at the S-expression level. The included file is parsed to instructions and inserted in place of the include directive. Error reports will always show which file the line and column refer to.
There are no special scoping rules. It is possible to define a label in a file and jump to it from another. Defines and register aliases (sym) also work cross-file.
Don't go too crazy with includes, it can get messy. Or do, I'm not your mom.
Instructions are written like this:
(<keyword> <args>... <conditional branches>...)
All instructions can be made conditional by appending
.<cond> to the keyword, i.e.
(j.ne :LABEL) means "jump if not equal".
These modifiers are mainly used by the assembler when translating conditional branches to executable code.
Note that the flags can only be tested immediately after the instruction that produced them, or after instructions that do not
affect flags (pseudo-instructions like
call etc). Instructions that can set flags first
clear all flags to make the result predictable.
Status flags can be saved to and restored from a register using the
ldf instructions. This can also be used to set
or test flags manually, but the binary format may change
Arguments are always ordered writes-first, reads-last.
This document uses the following notation for arguments:
REG- one of the registers (
SYM- a symbol defined as a register alias (e.g.
(sym x r0))
@SYM- access an object referenced by a handle. Handle is simply a numeric value stored in a register of some kind.
_- a special "register" that discards anything written to it. The "discard register" is used when you do not need the value and only care about side effects or status flags.
CONST- name of a constant defined earlier in the program (e.g.
(def SCREEN_WIDTH 640))
NUM- literal values
-45.6789. For now, you must use literals with a period to enter float literals, integers will not be converted to float when used in floating point instructions!
'🐁'. Supports unicode and C-style escapes. Use
\\for a literal backslash.
"str"- a double-quoted string (
"ahoj\n"). Supports unicode and C-style escapes. Use
\\for a literal backslash.
:LABEL- label name
PROC- routine name
PROC/A- routine name with arity (number of arguments)
The different ways to specify a value can be grouped as "reads" and "writes":
RW- intersection of the two sets, capable of reading and writing:
@sym) can be read or written as if they were a register, but only if the referenced object supports it.
Other objects may produce a runtime fault or set the INVALID flag.
The object syntax is also used to read values yielded by a generator-like coroutine.
In the instruction lists below, I will use the symbols
Rd for reads,
Wr for writes,
RW for read-writes, and
@Obj for object handles,
with optional description after an apostrophe, such as:
(add Wr'dst Rd'a Rd'b).
Some instructions use bit offsets and widths. Width is directly attached to the opcode (e.g.
offsets are attached at the respective arguments after a colon:
(ld16 r0:8 r1:32) -
load 16 bits, starting at offset 32 of
r0, starting at offset 8. The rest if the register is not affected.
Quite often you will want to do some calculations at compile time, for example to create a constant whose value depends on another,
or to compose a binary value using bit shifts and masking. This can be done in any place that expects an input operand (
The syntax for this is as follows:
(ld r0 (=add 123 456))
The expressions can be nested. The equals sign is not required in the inner levels, and the output operand must be omitted (the compiler inserts a placeholder register there during evaluation):
(def WIDTH 1024) (def HALFW_MAX (=sub (div WIDTH 2) 1))
Almost any instruction can be evaluated this way, excluding instructions that perform IO, work with the screen, objects etc. Instructions that are not compile-time evaluation safe will produce a compile error.
There are several limitations to consider:
(op Wr ...)can be used. The
Wroperand is inserted automatically and must NOT be specified!
Conditonal branches are written like this:
invalflag is to be checked, it should be done before checking e.g.
nz, which is, incidentally, true by default, because most flags are cleared by instructions that affects flags.
elsecan be used as a final choice of branch that will always be taken.
A routine is defined as:
(proc <name>/<arity> instructions...)
nameis a unique routine name
arityis the number of arguments it takes, e.g.
Or, with named arguments:
(proc <name> <arguments>... instructions...)
Arguments are simply aliases for the argument registers that can then be used inside the routine.
Here is an example routine to calculate the factorial of
(proc fac/1 (cmp arg0 2 (eq? (ret 2))) (sub r0 arg0 1) (call fac r0) (mul r0 arg0 res0) (ret r0) )
It can also be written like this:
(proc fac num ... )
...or by specifying both the arity and argument names:
(proc fac/1 num ... )
Croissant implements something that could be called "pre-emptive coroutines". They do not provide any performance gain, but add asynchronicity to the program, and can work as generators!
There is no true parallelism, it is difficult to implement safely and efficiently with a global state.
Any procedure can be used as a coroutine.
A coroutine is created using the
spawn instruction, which produces an object handle.
(spawn r0 do_stuff 1 2 3)
At this point, the program is evenly divided between the original and the coroutine "thread".
The spawned coroutine is scheduled to run immediately after being spawned.
Coroutines take turns to execute the program. The scheduling interval can be configured.
Control can be given up using the
yield instruction; for example, when waiting for a mutex. This happens automatically when
sleep instruction is invoked.
Take care when working with objects, resources and global registers: you can get race conditions
with coroutines. Use atomic instructions (
bfcas…) to implement synchronization.
casXX instruction is very powerful: you can use one bit of a register as a mutex and the rest of it to store some useful data.
You can also use one register for up to 64 mutexes.
Remember to only use global registers (or buffer items) as mutexes:
g15. Each coroutine has its own set of regular registers.
Another way to avoid race conditions is to use critical sections. Context switch (switching between active coroutines) is forbidden in a critical section. Try to keep critical sections as short as possible, since they can distort sleep times and cause other similar problems.
(crit-begin) ... (crit-end)
A safer way is to use the "critical block", which expands to the same, but also detects some common bugs at compile time, like trying to jump out of the critical section.
(crit ... )
Critical section nesting is allowed, but probably a bug.
A coroutine can "yield a value" by invoking the
yield instruction with an operand. This can be done any number of times.
The coroutine is blocked until the value is consumed by someone. To consume a yielded value, read the coroutine object handle:
(spawn r5 foo) (ld r0 @r5) ; read a yielded value
Caution!!! Due to the way this is implemented, the instruction that tries to use a yielded value might partially execute before detecting that it is blocked, and will be retried when the thread is next scheduled to run. This has the consequence that if something like a object handle is read by the same instruction, it may be read multiple times while waiting for the yielded value. It is recommended to never combine reads of a yielded value with reads of other object handles.
join instruction with a coroutine object handle to wait for its completion.
A coroutine completes by calling
ret at its top level. This naturally means that a coroutine can return values!
The returned values are placed in the result registers, just like with the
(spawn r5 foo) ; ... (join @r5) ; res0-res15 now contain return values
Crsn instruction set is composed of extensions.
Extensions can define new instructions as well as new syntax, so long as it's composed of valid S-expressions.
These are defined as part of the built-in instruction set (see below).
Skipping across conditional branches may have surprising results - conditional branches are expanded to a varying number of skips and conditional instructions by the assembler. Only use skips if you really know what you're doing.
Jumping to a label is always safer than a manual skip.
Infinite loops are a very common construct, so there is special syntax added for them.
These are converted by the assembler to an anonymous label and a jump to it.
(loop ...ops )
If you want to have a convenient jump target, give the loop a name. This lets you easily "break" and "continue" by jumping to the labels.
(loop :label ...ops )
(:label) ...ops (j :label) (:label-end)
; Do nothing (nop) ; Stop execution (halt) ; Define a register alias. ; The alias is only valid in the current routine or in the root of the program. ; However, if the register is a global register, then the alias is valid everywhere. (sym SYM REG) ; Define a constant. These are valid in the entire program following the definition point, unless ; un-defined. Constants are evaluated and assigned at compile time, the program control flow has no ; effect. ; Value must be known at compile time. (def CONST VALUE) ; Mark a jump target. (:LABEL) ; Numbered labels (:#NUM) ; Mark a far jump target (can be jumped to from another routine). ; This label is preserved in optimized code. (far :LABEL) ; Jump to a label (j :LABEL) ; Jump to a label that can be in another function (fj :LABEL) ; Skip backward or forward (s Rd) ; Copy a value (ld Wr Rd) ; Copy lower XX bits (the rest is untouched). ; Offsets can be specified to work with arbitrary bit slices (ldXX RW:dst_offset Rd:src_offset) (ldXX Wr Rd:dst_offset Rd:src_offset) ; Copy a value N times. This is useful when used with stream handles or buffers. (ldn Wr Rd'src Rd'count) ; Write a sequence of values, or all codepoints from a string, into the destination. ; This is most useful with object handles, such as a buffer or @cout. ; Functionally, this instruction is equivalent to a sequence of "ld" (lds Wr (Rd...)) ; example - (lds @cout (65 66 67)) (lds Wr "string") ; Some objects can be used as the source for "lds": ; - @cin = read all to EOF or the first fault (invalid utf8) ; - @buffer = read all items in a buffer, first to last, without consuming it (lds Wr @Obj) ; Exchange two register's values (xch RW RW) ; Exchange XX bits in two registers ; Offsets can be specified to work with arbitrary bit slices (xchXX RW:offset RW:offset) ; Compare and swap; atomic instruction for coroutine synchronization ; - The "Equal" flag is set on success, use it with conditional branches or conditional execution. ; - The new value is not read until the comparison passes and it is needed. ; This behavior may matter for side effects when used with object handles. (cas RW Rd'expected Rd'new) ; Compare and swap a bit slice; atomic instruction for coroutine synchronization ; See (cas) above for more info. ; ; Offsets can be specified to work with arbitrary bit slices (casXX RW:offset Rd:offset'expected Rd:offset'new) ; Store status flags to a register (stf Wr) ; Load status flags from a register (ldf Rd) ; Mark a routine entry point (call target). (routine PROC) (routine PROC/A) ; Call a routine with arguments. ; The arguments are passed as argX. Return values are stored in resX registers. (call PROC Rd...) ; Spawn a coroutine. The handle is stored in the output register. (spawn Wr'handle PROC Rd...) ; Exit the current routine (or coroutine) with return values (ret Rd...) ; Yield control from a coroutine (use when waiting for a mutex to give control early to ; other coroutines that might be holding it) (yield) ; Yield a value generated by a coroutine. Gives up control and blocks until the value is consumed. ; Conversely, an instruction that tries to read a yielded value using the object handle is blocked ; until such value becomes available. (yield Rd'value) ; Wait for a coroutine to complete, read its return values and delete it. (join @Obj) ; Begin a critical section (no context switch allowed) (crit-begin) ; End a critical section (crit-end) ; Shortcut to define a critical section (crit ...ops ) ; Generate a run-time fault with a debugger message (fault) (fault message) (fault "message text") ; Deny jumps, skips and run across this address, producing a run-time fault with a message. (barrier) (barrier message) (barrier "message text") ; Block barriers are used for routines. They are automatically skipped in execution ; and the whole pair can be jumped *across*. ; The label can be a numeric or string label, its sole purpose is tying the two together. They must be unique in the program. (barrier-open LABEL) (barrier-close LABEL) ; Set coroutine scheduler timeslice (in microseconds). Set to zero to disable preemption. (rt-opt RT_TIMESLICE Rd'usec)
This module makes heavy use of status flags.
Many instructions have two forms:
; Test properties of a value - zero, positive, negative (tst SRC) ; Compare two values. Sets EQ, LT, GT, and Z, POS and NEG if the values equal (cmp Rd Rd) ; Check if a value is in a range (inclusive). ; Sets the EQ, LT and GT flags. Also sets Z, POS and NEG based on the value. (rcmp Rd'val Rd'start Rd'end) ; Get a random number (rng Wr) ; the value will fill all 64 bits of the target (rng Wr Rd'max) ; 0 to max, max is inclusive (rng Wr Rd'min Rd'max) ; min to max, both are inclusive ; Add A+B (add Wr Rd Rd) (add RW Rd) ; Subtract A-B (sub Wr Rd Rd) (sub RW Rd) ; Multiply A*B (mul Wr Rd Rd) (mul RW Rd) ; Divide A/B (div Wr Rd Rd'divider) (div RW Rd'divider) ; Divide and get remainder ; Both DST and REM are output registers (divr Wr'result Wr'remainder Rd Rd'divider) (divr RW Wr'remainder Rd'divider) ; Get remainder A%B ; This is equivalent to (divr _ REM A B), ; except status flags are updated by the remainder value (mod Wr Rd Rd'divider) (mod RW Rd'divider) ; Get abs value (abs Wr Rd) (abs RW) ; Get signum (sgn Wr Rd) (sgn RW) ; Power - e.g. (pow r0 2 8) is 256 (pow Wr Rd Rd'pow) ; Swap the 32-bit halves of a value ; 0x01234567_89abcdef -> 0x89abcdef_01234567 (sw32 Wr Rd) (sw32 RW) ; Swap 16-bit halves of each 32-bit part ; 0x0123_4567_89ab_cdef -> 0x4567_0123_cdef_89ab (sw16 Wr Rd) (sw16 RW) ; Swap bytes in each 16-bit part ; 0x01_23_45_67_89_ab_cd_ef -> 0x23_01_67_45_ab_89_ef_cd (sw8 Wr Rd) (sw8 RW) ; Reverse endian (byte order) (rev Wr Rd) (rev RW) ; Reverse bit order (rbit Wr Rd) (rbit RW) ; Count leading zeros (clz Wr Rd) (clz RW) ; Count leading zeros in the lower XX bits ; Offsets can be specified to work with arbitrary bit slices (clzXX Wr Rd:src_offs) (clzXX RW:src_offs) ; Count leading ones (clo Wr Rd) (clo RW) ; Count leading ones in the lower XX bits ; Offsets can be specified to work with arbitrary bit slices (cloXX Wr Rd:src_offs) (cloXX RW:src_offs) ; Sign extend a XX-bit value to 64 bits, XX in range 1..63) (seXX Wr Rd) (seXX RW) ; AND A&B (and Wr Rd Rd) (and RW Rd) ; OR A|B (or Wr Rd Rd) (or RW Rd) ; XOR A&B (xor Wr Rd Rd) (xor RW Rd) ; CPL ~A (negate all bits) (cpl DST A) (cpl DST) ; Rotate right (wrap around) (ror Wr Rd Rd) (ror RW Rd) ; Rotate left (wrap around) (rol Wr Rd'value Rd'count) (rol RW Rd'count) ; Logical shift right (fill with zeros) (lsr Wr Rd Rd'count) (lsr RW Rd'count) ; Logical shift left (fill with zeros) (lsl Wr Rd Rd'count) (lsl RW Rd'count) ; Arithmetic shift right (copy sign bit) (asr Wr Rd Rd'count) (asr RW Rd'count) ; Arithmetic shift left (this is identical to `lsl`, added for completeness) (asl Wr Rd Rd'count) (asl RW Rd'count) ; Delete an object by its handle. Objects are used by some extensions. (del @Rd)
The arithmetics module has support for floating point values. There are some gotchas though:
(itf r0 1.0) ; NO!!! it is already float, what are you doing (ld r0 1.0) ; okay (itf r0 1) ; also okay (fmul r0 2) ; NO!!!!!!!!!!!!! 2 is not float (mul r0 2.0) ; ALSO NO!!!!!!!!!!!!! mul is not a float instruction! (fmul r0 2.0) ; good
You have to be a bit careful, that's all.
Here's an abridged summary of the floating point submodule:
(most of these support the shorthand version too -
RW in place of
; Convert int to float (itf Wr Rd) ; Convert float to int (round) (fti Wr Rd) ; Convert float to int (ceil) (ftic Wr Rd) ; Convert float to int (floor) (ftif Wr Rd) ; Test properties of a float ; Set flags: ; NaN -> invalid ; Infinities -> overflow ; Positive, negative, zero (ftst Rd) ; Float compare (fcmp Rd Rd) ; Float range test (fcmpr Rd Rd'min Rd'max) ; FloatRng. Unlike rng, frng is exclusive in the higher bound (frng Wr Rd'min Rd'max) ; --- Basic float arith --- ; Float add (fadd Wr Rd Rd) ; Float subtract (fsub Wr Rd Rd) ; Float multiply (fmul Wr Rd Rd) ; Float power (fpow Wr Rd Rd'pow) ; Float root (froot Wr Rd Rd'root) ; Float hyp - sqrt(a*a + b*b) (fhyp Wr Rd Rd) ; Float divide (fdiv Wr Wr'rem Rd'a Rd'div) ; Float modulo (fmod Wr Rd'a Rd'div) ; Float abs value (fabs Wr Rd) ; Float signum (returns -1 or 1) (fsgn Wr Rd) ; --- Basic trig --- ; Float sine (fsin Wr Rd) ; Float arcsine (fasin Wr Rd) ; Float cosine (fcos Wr Rd) ; Float arccosine (facos Wr Rd) ; Float tangent (ftan Wr Rd) ; Float arctangent (fatan Wr Rd) ; Float 2-argumenmt arctangent (fatan2 Wr Rd'y Rd'x) ; Float cotangent (fcot Wr Rd) ; Float arccotangent (facot Wr Rd) ; --- Hyperbolic trig --- ; Float hyperbolic sine (fsinh Wr Rd) ; Float hyperbolic arcsine (fasinh Wr Rd) ; Float hyperbolic cosine (fcosh Wr Rd) ; Float hyperbolic arccosine (facosh Wr Rd) ; Float hyperbolic tangent (ftanh Wr Rd) ; Float hyperbolic arctangent (fatanh Wr Rd) ; Float hyperbolic cotangent (fcoth Wr Rd) ; Float hyperbolic arccotangent (facoth Wr Rd)
Wow, thats a lot. I didn't test many of these yet. There may be bugs.
There are also some pre-defined constants:
This module defines dynamic size integer buffers.
A buffer needs to be created using one of the init instructions:
; Create an empty buffer and store its handle into a register (mkbf Wr) ; Create a buffer of a certain size, filled with zeros. ; COUNT may be a register or an immediate value (mkbf Wr Rd:count) ; Create a buffer and fill it with characters from a string (unicode code points) (mkbf Wr "string") ; Create a buffer and fill it with values. (mkbf Wr (Rd...))
Buffers can be as stacks or queues by reading and writing the handle (e.g.
(ld @buf 123)).
The behavior of reads and writes is configurable per stack, and can be changed at any time.
The default mode is forward queue.
This feature may be a bit confusing at first, but it is extremely powerful.
One consequence of this feature is that
(ld @buf @buf) will move items from one end to the other
in one or the other direction (queue mode), or do nothing at all (stack mode).
; Set buffer IO mode ; Mode is one of: ; - BFIO_QUEUE (1) ; - BFIO_RQUEUE (2) ; - BFIO_STACK (3) ; - BFIO_RSTACK (4) (bfio @Obj MODE)
Primitive buffer ops (position is always 0-based)
; Get buffer size (bfsz Wr @Obj) ; Read from a position (bfrd Wr @Obj Rd:index) ; Write to a position (bfwr @Obj Rd:index Rd) ; Insert at a position, shifting the rest to the right (bfins @Obj Rd:index Rd) ; Remove item at a position, shifting the rest to the left to fill the empty space (bfrm Wr @Obj Rd:index) ; Buffer value compare and swap; atomic instruction for coroutine synchronization. ; - The "Equal" flag is set on success, use it with conditional branches or conditional execution. ; - The new value is not read until the comparison passes and it is needed. ; This behavior may matter for side effects when used with object handles. ; ; This instruction is useful when more than one lock is needed and they are stored in a buffer at well known positions. ; Naturally, the buffer must not be mutated in other ways that would undermine the locking. ; ; If an index just outside the buffer is used, the value is read as zero the position is created (if zero was expected). (bfcas @Obj Rd:index Rd'expected Rd'new)
Whole buffer manipulation:
; Resize the buffer. Removes trailing elements or inserts zero to match the new size. (bfrsz @Obj Rd:len) ; Reverse a buffer (bfrev @Obj) ; Append a buffer (bfapp @Obj @Obj:other) ; Prepend a buffer (bfprep @Obj @Obj:other)
Stack-style buffer ops:
; Push (insert at the end) (bfpush @Obj Rd) ; Pop (remove from the end) (bfpop Wr @Obj) ; Reverse push (insert to the beginning) (bfrpush @Obj Rd) ; Reverse pop (remove from the beginning) (bfrpop Wr @Obj)
To delete a buffer, use the
del instruction -
This module uses the minifb rust crate to provide a framebuffer with key and mouse input.
Colors use the
#RRGGBB hex format.
If input events are required, then make sure to periodically call
This may not be needed if the auto-blit function is enabled and the display is regularly written.
The default settings are 60 FPS and auto-blit enabled.
NOTE: Logging can significantly reduce crsn run speed. Make sure the log level is at not set to "trace" when you need high-speed updates, such as animations.
; Initialize the screen (opens a window) (sc-init WIDTH HEIGHT) ; Set pixel color (sc-wr Rd'x Rd'y Rd'color) ; legacy name: sc-px ; Get pixel color (sc-rd Wr'color Rd'x Rd'y) ; Set screen option. Constants are pre-defined. ; - SCREEN_AUTO_BLIT (1) ... auto-blit (blit automatically on pixel write when needed to achieve the target FPS) ; - SCREEN_FPS (2) ......... frame rate ; - SCREEN_UPSCALE (3) ..... upscaling factor (big pixels). ; Scales coordinates for mouse and pixels and increases pixel area (sc-opt OPTION VALUE) ; Blit (render the pixel buffer). ; This function also updates key and mouse states and handles the window close button (sc-blit) ; Blit if needed (when the auto-blit function is enabled) (sc-blit 0) ; Update key and mouse state, handle the window close button (sc-poll) ; Read mouse position into two registers. ; Sets the overflow flag if the cursour is out of the window (sc-mouse X Y) ; Check key status. Keys are 0-127. Reads 1 if the key is pressed, 0 if not. ; A list of supported keys can be found in the extension source code. ; Run the example screen_keys.scn to interactively check key codes. (sc-key Wr'pressed Rd'num) ; Check mouse button state. ; 0-left, 1-right, 2-middle (sc-mbtn Wr'pressed Rd'btn)
Available constants provided by the module:
; Fill a rectangle (sc-rect Rd'x Rd'y Rd'w Rd'h Rd'color) ; Erase the screen (fill with black) (sc-erase) ; Fill with a custom color (sc-erase Rd'color)
(ld r0 @cin)).
End of stream is reported by the 'eof' status flag when a stream is read or written.
You can use these special handles in almost all instructions:
(cmp @cin 'y' (eq? (ld @cout '1')) (ne? (ld @cout '0')))
When you compile a program using such handles, you will get a strange looking assembly:
0000 : (ld @0x6372736e00000001 72) 0001 : (ld @0x6372736e00000001 101) 0002 : (ld @0x6372736e00000001 108)
These are unique constants assigned to the streams at compile time. They are not meant to be used
directly, but the value can be obtained by simply leaving out the '@' sign:
(ld r0 cin).
That can be useful when these stream handles need to be passed to a function. Obviously this makes
more sense when there are different kinds of streams available, not just these two default ones.