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// written by Paul Baxter
/**
* @file autoloader.cpp
* @author Paul Baxter
* @brief Implementation of the Commodore 64 autostart bootstrap loader generator.
*/
#include "autoloader.h"
#include <cctype>
#include <stdexcept>
/**
* @brief Generates and patches the Commodore 64 autostart bootstrap loader binary.
*
* This function constructs a raw byte array representing a 6502 machine code routine
* designed to run at address $0102. It suppresses interrupts, interacts with C64 KERNAL
* routines (`RESTOR`, `STROUT`, `SETNAM`, `SETLFS`, `LOAD`) to load the specified
* program from disk silently, restores system vectors, and jumps directly to the
* target program entry address.
*
* @param filename The target PRG filename to load from disk.
* @param addr The target execution/load address of the main program.
* @return std::vector<uint8_t> The complete, ready-to-write LOADER.PRG binary array.
*/
std::vector<uint8_t> CreateAutoLoader(const std::string& filename, uint16_t addr) {
std::vector<uint8_t> loader_code = {
// ----------------------------------------------------
// Autostart Bootstrap Loader (* = $0102)
// ----------------------------------------------------
0x02, 0x01, // [0, 1] preset load address
0xA9, 0x7F, // [2, 3] lda #$7F (suppress irq & nmi)
0x8D, 0x0D, 0xDC, // [4, 5, 6] sta CIAICR
0x8D, 0x0E, 0xDC, // [7, 8, 9] sta CIACRA
0x20, 0x8A, 0xFF, // [10, 11, 12] jsr RESTOR
0xA9, 0x36, // [13, 14] lda #36 (<MSG)
0xA0, 0x01, // [15, 16] ldy #01 (>MSG)
0x20, 0x1E, 0xAB, // [17, 18, 19] jsr STROUT
0xA9, 0x00, // [20, 21] lda #NAMELEN ([21] is the length byte to update)
0xA2, 0x37, // [22, 23] ldx #37 (<NAME)
0xA0, 0x01, // [24, 25] ldy #01 (>NAME)
0x20, 0xBD, 0xFF, // [26, 27, 28] jsr SETNAM
0xA5, 0xBA, // [29, 30] lda FA (device number)
0xAA, // [31] tax
0xA8, // [32] tay
0x20, 0xBA, 0xFF, // [33, 34, 35] jsr SETLFS
0xA9, 0x00, // [36, 37] lda #00 (load mode)
0x85, 0x9D, // [38, 39] sta MSGFLG (suppress 'searching for' msg)
0x20, 0xD5, 0xFF, // [40, 41, 42] jsr LOAD
0xA9, 0x81, // [43, 44] lda #$81 (restore irq & nmi)
0x8D, 0x0D, 0xDC, // [45, 46, 47] sta $DC0D
0x8D, 0x0E, 0xDC, // [48, 49, 50] sta $DC0E
0x4C, 0x00, 0x00, // [51, 52, 53] jmp $C000 (start loaded program)
// ----------------------------------------------------
// Data & Strings (MSG and NAME)
// ----------------------------------------------------
0x20, // [54] MSG: .byte " "
0x01, 0x01, 0x01, // [55, 56, 57] NAME START HERE
0x01, // [58]
// ----------------------------------------------------
// Fill Padding (196 bytes of $01)
// ----------------------------------------------------
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
0x01, 0x01, 0x01, 0x01, 0x01
};
// 1. Update the name length byte (at index 21)
loader_code[21] = static_cast<uint8_t>(filename.length());
// 2. Set the load address (jumps to target program entry at bytes 52-53)
auto lo = static_cast<uint8_t>(addr & 0xFF);
auto hi = static_cast<uint8_t>((addr >> 8) & 0xFF);
loader_code[52] = lo;
loader_code[53] = hi;
// 3. Overwrite the name bytes starting at index 55
constexpr size_t name_offset = 55;
for (size_t i = 0; i < filename.length(); ++i) {
loader_code[name_offset + i] = static_cast<uint8_t>(std::toupper(filename[i]));
}
// 3. Add the null terminator right after the filename
loader_code[name_offset + filename.length()] = 0x00;
return loader_code;
}