arm_disasm_explained.cpp

// pld.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <stdio.h>
#include <string.h>
#include <stdint.h>

struct PLD {
	unsigned addr_mode : 12;
	unsigned z0 : 4; // 1111
	unsigned Rn : 4;
	unsigned z1 : 3; // 101
	unsigned U : 1;
	unsigned z2 : 1; // 1
	unsigned I : 1;
	unsigned z3 : 2; // 01
	unsigned cond : 4; // 1111
};
static_assert(sizeof(PLD) == 4, "Invalid size");

void print_bits(uint32_t ins)
{
	for (int i = 0; i < (sizeof(uint32_t) * 8); i++)
	{
		if (i == 12 ||
			i == 16 ||
			i == 20 ||
			i == 23 ||
			i == 24 || 
			i == 25 ||
			i == 26 ||
			i == 28)
			printf(":");

		printf("%i", ins & 0x01);

		ins = ins >> 1;
	}
}

int main()
{
	PLD ins;

	// the "trick" instruction i posted on my blog
	// A normal PLD is commented
	unsigned char uc[4] = { 0x02, 0xDF, 0xDF, 0xF5 /* 0x02, 0xF0, 0xDF, 0xF5 */ };
	memcpy(&ins, uc, 4);

	printf("pld    = ");
	print_bits(*(uint32_t*)&ins);
	printf("\n");

	PLD output;
	output.addr_mode = 0b101010101010;
	output.z0 = 0b0000;						// Making this *not* 1111 makes the instruction malformed.
	output.Rn = 0b1111;
	output.z1 = 0b101;
	output.U = 0b0;
	output.z2 = 0b1;
	output.I = 0b1;
	output.z3 = 0b01;
	output.cond = 0b1111;

	printf("output = ");
	print_bits(*(uint32_t*)&output);
	printf("\n");

	memcpy(uc, &output, 4);

	printf("uc = { 0x%X, 0x%X, 0x%X, 0x%X }\n", uc[0], uc[1], uc[2], uc[3]);

	return 0;
}