diff --git a/README.md b/README.md
index 9033054..e52f45a 100644
--- a/README.md
+++ b/README.md
@@ -23,4 +23,4 @@ _in which I attempt to use a different language every day_
 9. Picat
 10. Hy
 11. GNU Octave
-12. ??
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+12. fasmg macros
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diff --git a/d12/README.md b/d12/README.md
index 3556f78..d676d5e 100644
--- a/d12/README.md
+++ b/d12/README.md
@@ -1,11 +1,20 @@
 # Day 12
 
-|                   |                   |
-|-------------------|-------------------|
-| Language          | ??                |
-| Input Method      | ??                |
-| Output Method     | ??                |
+|                   |                                               |
+|-------------------|-----------------------------------------------|
+| Language          | fasmg macros                                  |
+| Input Method      | File                                          |
+| Output Method     | File (raw 16-bit little endian binary number) |
 
 ## Running
+```sh
+fasmg -i'include "day12.inc"' <path to input file>
+```
 
-## Notes
\ No newline at end of file
+## Notes
+This relies on a particular property of real inputs which doesn't hold for the example, so this will not provide the correct answer for
+the example input.
+
+The output will be left in a binary file with the same name as the input file but with the extension removed. The output file will be a
+2-byte binary, which is the puzzle answer as a 16-bit little endian number. Note that the output file will be left in the same directory
+as the input file, not the current working directory.
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diff --git a/d12/day12.inc b/d12/day12.inc
new file mode 100644
index 0000000..db575a3
--- /dev/null
+++ b/d12/day12.inc
@@ -0,0 +1,78 @@
+macro count_hashes: res*, line*
+    match ## , line
+        res = 2
+    else match #., line
+        res = 1
+    else match .#, line
+        res = 1
+    else match .., line
+        res = 0
+    else match # rest , line
+        local recursive_res
+        count_hashes recursive_res, rest
+        res = 1 + recursive_res
+    else match . rest , line
+        count_hashes res, rest
+    end match
+end macro
+
+macro sum_areas: res*, count*, line*
+    match n rest, line
+        local tmp
+        sum_areas tmp, count, rest
+        repeat 1, i:count
+            res = tmp + (area#i * n)
+        end repeat
+    else 
+        repeat 1, i:count
+            res = area#i * line
+        end repeat
+    end match
+end macro
+
+ans = 0
+
+macro ? line
+    match num:, line
+        current_shape = num
+        repeat 1, i:current_shape
+            area#i = 0
+        end repeat
+    else match # rest, line
+        local tmp
+        count_hashes tmp, rest
+        repeat 1, i:current_shape
+            area#i = area#i + tmp + 1
+        end repeat
+    else match . rest, line
+        local tmp
+        count_hashes tmp, rest
+        repeat 1, i:current_shape
+            area#i = area#i + tmp
+        end repeat
+    else match dims: rest, line
+        virtual at 0
+            db `dims
+            load tmp:byte from $-5
+            a = 10 * (tmp - $30)
+            load tmp:byte from $-4
+            a = a + (tmp - $30)
+            load tmp:byte from $-2
+            b = 10 * (tmp - $30)
+            load tmp:byte from $-1
+            b = b + (tmp - $30)
+        end virtual
+        
+        local area
+        sum_areas area, 0, rest
+        if area <= (a * b)
+            ans = ans + 1
+        end if
+    else
+        line
+    end match
+end macro
+
+postpone
+    dw ans
+end postpone
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