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Brendan Punsky eda4921c65 rexcode/arm32: six operands, and a coprocessor block that was wrong in four ways
An instruction carried four operands. The coprocessor transfers need
six -- `mcr p15, #0, r0, c0, c0, #0` -- and dropping the tail is not
merely printing short: an opc2 that is not zero encoded as zero, which
is a different instruction. Six operands do not fit alongside the rest
in 64 bytes, since Operand is 11 and Memory cannot go below 8, so
Instruction is 128 now and straddles two cache lines rather than one.

With room for them, the block itself turns out to have been wrong
throughout. CDP, MCR and MRC took CRd from bits 19:16, which is CRn;
CRd is 15:12. MCR and MRC read a four-bit opc1 where theirs is the
three bits at 23:21. Neither they nor CDP read opc2 at all. MCRR and
MRRC dropped CRm.

LDC and STC were worse: they took CRd from the base register's field,
read the offset with the halfword-load encoding rather than as an imm8
counting words, and their base pattern named P=0 W=0, which is the
unindexed form whose syntax is `{option}` and not what they printed.
They now have the offset, pre-indexed and post-indexed forms, with U
left out of the mask because it is the offset's sign and belongs to the
memory operand. The N bit asks the coprocessor for the long transfer
and the syntax spells that as an L, so LDCL, STCL, LDC2L and STC2L are
mnemonics of their own.

None of this is reachable from the table sweep, which decodes each
form's canonical word with every variable field zero -- for these that
means p0, which ARMv8 reserves and no assembler will take. So twelve
hand-picked words go in the pipeline test instead, checked against the
text an assembler accepts for them, including a p15 system-register
read.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018UmHLRF11EoWwNWCJ7JGaA
2026-08-29 10:07:00 -04:00
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Odin logo
The Data-Oriented Language for Sane Software Development.


The Odin Programming Language

Odin is a general-purpose programming language with distinct typing, built for high performance, modern systems, and built-in data-oriented data types. The Odin Programming Language, the C alternative for the joy of programming.

Website: https://odin-lang.org/

package main

import "core:fmt"

main :: proc() {
	program := "+ + * 😃 - /"
	accumulator := 0

	for token in program {
		switch token {
		case '+': accumulator += 1
		case '-': accumulator -= 1
		case '*': accumulator *= 2
		case '/': accumulator /= 2
		case '😃': accumulator *= accumulator
		case: // Ignore everything else
		}
	}

	fmt.printf("The program \"%s\" calculates the value %d\n",
	           program, accumulator)
}

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