From a71be16a93ae0aa3df9fef15d14a0e157d8b9af6 Mon Sep 17 00:00:00 2001
From: pjht <terppa05@gmail.com>
Date: Mon, 14 Nov 2022 11:01:03 -0600
Subject: [PATCH] Initial commit

---
 .gitignore   |  3 ++
 build_rom.sh |  5 ++++
 rom.68k      | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 rom.ld       |  5 ++++
 4 files changed, 92 insertions(+)
 create mode 100644 .gitignore
 create mode 100755 build_rom.sh
 create mode 100644 rom.68k
 create mode 100644 rom.ld

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9cd5496
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+*.bin
+*.elf
+*.o
diff --git a/build_rom.sh b/build_rom.sh
new file mode 100755
index 0000000..c2181c2
--- /dev/null
+++ b/build_rom.sh
@@ -0,0 +1,5 @@
+#! /bin/bash -x
+m68k-elf-as -m68010 --register-prefix-optional -o rom.o rom.68k
+m68k-elf-ld -z max-page-size=1 -T rom.ld -o rom.elf rom.o
+m68k-elf-objcopy -O binary rom.elf rom.bin
+
diff --git a/rom.68k b/rom.68k
new file mode 100644
index 0000000..b5a9d22
--- /dev/null
+++ b/rom.68k
@@ -0,0 +1,79 @@
+  .long fakestack, start
+start:
+  move.b #0x1, d0 | Find the ROM card
+  bra.b find_first_card
+romfindret:
+  move.l a0, a6 | Save the ROM card IO base in a6 for later
+  lea (0xF0, a6), a7 | Set up the stack at the end of the ROM's IO space RAM
+  bsr.b find_largest_ram | Find the largest RAM card and put the IO base in a5
+  move.l a0, a5
+  move.l d0, d7
+  move.b #0x4, d0 | Find a storage card and put the IO base in a4
+  bsr.b find_first_card
+  move.l a0, a4
+  | Transfer the bootsector load code to the ROM's
+  | built in RAM at the start of it's IO space
+  move.w #(ramcode_end - ramcode), d0 | Put the length of the ramcode in d0
+  move.w #ramcode, a0 | Put the address of the ramcode in a0
+  move.l a6, a1 | Put the start of the ROM's IO space RAM in a0
+  ramcode_loop:
+  move.b (a0)+, (a1)+ | Transfer a byte of ramcode to the ROM's IO space RAM
+  dbra d0, ramcode_loop | Loop back if there is more to transfer
+  jmp (a6) | Jump to the ramcode
+  stop #0x2700
+
+ramcode:
+  move.b #0x0, (0xFE, a6) | Disable the ROM
+  move.l #0x1, (a5) | Enable the RAM at base 0x0
+  move.l d7, a7
+  | Load sector 0 to 0x0
+  move.l #0x0, (a4) | Set the sector number to 0
+  | Transfer 0x100 (256) bytes from the storage card's data register to 0x0
+  move.w #0x0, a1 | Load a1, the destination address register, with 0x0
+  move.w #0xFF, d0 | Load d0 with the sector size - 1.
+  sector_loop:
+  move.b (4, a4), (a1)+ | Transfer a byte of sector data to the destination
+  dbra d0, sector_loop | Loop back if there is more to transfer
+  jmp (0x0).W | Jump to the loaded sector
+
+  stop #0x2700 
+ramcode_end:
+
+nop | Padding to make sure ramcode_end and find_first_card are different
+
+| Finds the first card with the type in d0.b, and returns it's IO base address in a0, or 0 if not found
+| Clobbers d1
+find_first_card:
+  move.l #0xff0000, a0 | a0 holds the address of the current card
+ffc_loop:
+  lea (0x100,a0), a0 | adda.l #$100,a0 ; Move to the next card
+  move.b (0xff, a0), d1 | Load the type of the card into d1
+  beq.b ffc_done | If the type is 0 (empty slot), we have scanned all cards, so exit the loop
+  cmp.b d0, d1 | If the card is the type we want, return with the address in a0
+  beq.b ffc_done
+  bra.b ffc_loop | Loop back and check the next card
+ffc_done:
+  rts
+
+| Finds the largest RAM card, and returns it's IO base address in a0 and size in d0, or 0 if not found
+| Clobbers d1, a1
+find_largest_ram:
+  move.l #0x0, d0 | d0 holds the size of the largest RAM card found
+  move.w #0x0, a0 | a0 holds the address of the largest RAM card found
+  move.l #0xff0000, a1 | a1 holds the address of the current card
+flr_loop:
+  lea (0x100,a1), a1 | adda.l #$100,a0 ; Move to the next card
+  move.b (0xff, a1), d1 | Load the type of the card into d1
+  beq.b flr_done | If the type is 0 (empty slot), we have scanned all cards, so exit the loop
+  cmp.b #0x2, d1 | If the card isn't a RAM card, skip it
+  bne.b flr_loop
+  move.l (0x4, a1), d1 | Load the card's size into d1
+  cmp.l d0, d1 | If the current size is less than the largest size found, go back to the start of the loop
+  ble.b flr_loop
+  move.l d1, d0 | Store the size and address of the new largest card in d0 and a0
+  move.l a1, a0
+  bra.b flr_loop | Loop back and check the next card
+flr_done:
+  rts
+fakestack: 
+  .long romfindret
diff --git a/rom.ld b/rom.ld
new file mode 100644
index 0000000..539193c
--- /dev/null
+++ b/rom.ld
@@ -0,0 +1,5 @@
+SECTIONS {
+  . = 0x0;
+  _etext = .;
+  .text : { *(.text) }
+}