diff --git a/abi_ref b/abi_ref
new file mode 100644
index 0000000..173e25b
--- /dev/null
+++ b/abi_ref
@@ -0,0 +1,4 @@
+d0, d1, a0, and a1 are caller preserved
+d2-d7 and a2-a5 are callee preserved
+a6 is frame pointer
+a7 is stack pointer
diff --git a/rom.68k b/rom.68k
index f34448f..87de61f 100644
--- a/rom.68k
+++ b/rom.68k
@@ -4,11 +4,11 @@ _start:
   move.w #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
+  move.l a0, a2 | Save the ROM card IO base in a6 for later
   move.l #0x10000, a7 | Set up the stack at the end of the ROM card's RAM
-  bsr.w find_largest_ram | Find the largest RAM card and put the IO base in a5
-  move.l a0, a5
-  move.l d0, d7
+  bsr.w find_largest_ram | Find the largest RAM card and put the IO base in a3
+  move.l a0, a3
+  move.l d0, d2
   move.l #0x8100, a1
   bsr.w find_all_ram_cards
   move.l #0x8100, a0
@@ -20,20 +20,20 @@ romfindret:
   | 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
+  move.l a2, 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
+  jmp (a2) | Jump to the ramcode
 
 ramcode:
-  move.b #0x0, (0xF3, a6) | Disable the ROM
-  move.l #0x1, (a5) | Enable the RAM at base 0x0
-  cmpi.l #0x1000000, d7
+  move.b #0x0, (0xF3, a2) | Disable the ROM
+  move.l #0x1, (a3) | Enable the RAM at base 0x0
+  cmpi.l #0x1000000, d2
   blt.b sp_ok
-  move.l #0xff0000, d7
+  move.l #0xff0000, d2
   sp_ok:
-  move.l d7, a7
+  move.l d2, a7
   | Load sector 0 to 0x0
   move.l #0x0, (0x0, a4) | Set the sector number to 0
   move.l #0x1, (0x4, a4) | Set the sector count to 1
@@ -45,7 +45,6 @@ ramcode_end:
 nop | Padding to make sure ramcode_end and find_first_card are different
 
 | Finds the first card with the type in d0.w, 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:
@@ -59,7 +58,6 @@ 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
@@ -83,7 +81,6 @@ fakestack:
 
 | 1KB buffer in a1
 | Returns list length in d0
-| Clobbers a1, d1
 find_all_ram_cards:
   move.b #0, d0 | d0 holds the number of RAM cards found.
   move.l #0xff0000, a0 | a0 holds the address of the current card.
@@ -103,8 +100,8 @@ farc_done:
 | optimized bubble sort to sort RAM cards by size
 | Buffer in a0
 | Length in d0
-| Clobbers d1, d2, d3, d4, a1, a2
 sort_ram_cards:
+movem.l d2-d4/a2, -(a7) | Save calle-preserved registers
 move.b #0, d1 | d1 = newlen
 move.b #1, d2 | d2 = i
 move.l a0, a1 | a1 holds the pointer to the current pair
@@ -129,4 +126,5 @@ src_inner_loop_done:
 move.b d1, d0 | len = newlen
 cmp #1, d0 | if n > 1, branch to outer loop
 bgt.b sort_ram_cards
+movem.l (a7)+, d2-d4/a2 | Restore calle-preserved registers
 rts