diff --git a/src/assembler.c b/src/assembler.c
index 31a1cdf..44c1045 100644
--- a/src/assembler.c
+++ b/src/assembler.c
@@ -23,8 +23,20 @@ static ErrorInfo* tokenize(AssemblerState *state)
 
     // verify no instructions clash with header offset
     // if rom size is set, verify nothing overflows
+    // otherwise, check nothing overflows max rom size (1 MB)
 
     (void) state;
+
+#ifdef DEBUG_MODE
+    DEBUG("Dumping ASMLines:")
+    const ASMLine *temp = state->lines;
+    while (temp) {
+        DEBUG("- %-40.*s [%s:%02zu]", (int) temp->length, temp->data,
+              temp->filename, temp->original->lineno)
+        temp = temp->next;
+    }
+#endif
+
     return NULL;
 }
 
@@ -39,13 +51,13 @@ static ErrorInfo* resolve_defaults(AssemblerState *state)
     // TODO
 
     // if (!state.rom_size)
-            // set to max possible >= 32 KB, or error if too many instructions
-            // if (state.header.rom_size)
-                    // check reported rom size is <= actual rom size
+            // set to max possible >= 32 KB (max of instructions, offset, and state.header.rom_size)
 
     // if (!state.header.rom_size)
             // set to actual rom size using util's size_bytes_to_code()
 
+    state->rom_size = 8;
+
     (void) state;
     return NULL;
 }
diff --git a/src/assembler/errors.c b/src/assembler/errors.c
index e2a39f9..fe89515 100644
--- a/src/assembler/errors.c
+++ b/src/assembler/errors.c
@@ -76,7 +76,7 @@ ErrorInfo* error_info_create(
 
     einfo->type = err_type;
     einfo->desc = err_desc;
-    einfo->line = create_error_line(line);
+    einfo->line = line ? create_error_line(line) : NULL;
     return einfo;
 }
 
diff --git a/src/assembler/errors.h b/src/assembler/errors.h
index c7299f7..1045df6 100644
--- a/src/assembler/errors.h
+++ b/src/assembler/errors.h
@@ -23,7 +23,9 @@ typedef enum {
     ED_PP_DUPLICATE,
     ED_PP_NO_ARG,
     ED_PP_BAD_ARG,
-    ED_PP_ARG_RANGE
+    ED_PP_ARG_RANGE,
+    ED_PP_HEADER_RANGE,
+    ED_PP_DECLARE_RANGE
 } ASMErrorDesc;
 
 /* Strings */
@@ -42,7 +44,9 @@ static const char *asm_error_descs[] = {
     "multiple values for directive",
     "missing argument for directive",
     "invalid argument for directive",
-    "directive argument out of range"
+    "directive argument out of range",
+    "header offset exceeds given ROM size",
+    "declared ROM size in header exceeds actual size"
 };
 
 /* Structs */
diff --git a/src/assembler/preprocessor.c b/src/assembler/preprocessor.c
index 2eb35a6..bf41671 100644
--- a/src/assembler/preprocessor.c
+++ b/src/assembler/preprocessor.c
@@ -15,6 +15,7 @@
 #include "io.h"
 #include "parse_util.h"
 #include "../logging.h"
+#include "../rom.h"
 #include "../util.h"
 
 /* Helper macros for preprocess() */
@@ -49,6 +50,9 @@
 #define PARSER_BRANCH(arg_type, true_part, false_part)                        \
     if (CALL_PARSER_(arg_type)) {true_part} else {false_part}
 
+#define SAVE_LINE(target)                                                     \
+    target = line;
+
 #define BEGIN_DIRECTIVE_BLOCK                                                 \
     ssize_t first_ctr = -1;                                                   \
     if (0) {}
@@ -233,6 +237,7 @@ static ErrorInfo* build_asm_lines(
     const LineBuffer *root, const LineBuffer *source, ASMLine **head,
     ASMLine **tail, ASMInclude **includes)
 {
+    ErrorInfo *ei;
     ASMLine dummy = {.next = NULL};
     ASMLine *line, *prev = &dummy;
     const Line *orig, *next_orig = source->lines;
@@ -249,21 +254,16 @@ static ErrorInfo* build_asm_lines(
         line->next = NULL;
 
         if (IS_DIRECTIVE(line, DIR_INCLUDE)) {
-            ErrorInfo *ei;
             char *path = read_include_path(line);
             if (!path) {
                 ei = error_info_create(line, ET_INCLUDE, ED_INC_BAD_ARG);
-                asm_lines_free(line);
-                asm_lines_free(dummy.next);
-                return ei;
+                goto error;
             }
 
             if (path_has_been_loaded(path, root, *includes)) {
-                ei = error_info_create(line, ET_INCLUDE, ED_INC_RECURSION);
-                asm_lines_free(line);
-                asm_lines_free(dummy.next);
                 free(path);
-                return ei;
+                ei = error_info_create(line, ET_INCLUDE, ED_INC_RECURSION);
+                goto error;
             }
 
             DEBUG("- reading included file: %s", path)
@@ -271,9 +271,7 @@ static ErrorInfo* build_asm_lines(
             free(path);
             if (!incbuffer) {
                 ei = error_info_create(line, ET_INCLUDE, ED_INC_FILE_READ);
-                asm_lines_free(line);
-                asm_lines_free(dummy.next);
-                return ei;
+                goto error;
             }
 
             ASMInclude *include = malloc(sizeof(ASMInclude));
@@ -288,9 +286,7 @@ static ErrorInfo* build_asm_lines(
             if ((ei = build_asm_lines(root, incbuffer, &inchead, &inctail,
                                       includes))) {
                 error_info_append(ei, line);
-                asm_lines_free(line);
-                asm_lines_free(dummy.next);
-                return ei;
+                goto error;
             }
 
             prev->next = inchead;
@@ -307,6 +303,11 @@ static ErrorInfo* build_asm_lines(
     if (tail)
         *tail = prev;
     return NULL;
+
+    error:
+    asm_lines_free(line);
+    asm_lines_free(dummy.next);
+    return ei;
 }
 
 /*
@@ -342,6 +343,7 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
 
     ASMLine dummy = {.next = state->lines};
     ASMLine *prev, *line = &dummy, *next = state->lines, *condemned = NULL;
+    const ASMLine *rom_size_line = NULL, *rom_declsize_line = NULL;
     const char *directive;
 
     while ((prev = line, line = next)) {
@@ -364,6 +366,7 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
                 USE_PARSER(uint32_t, rom_size)
             })
             VALIDATE(size_bytes_to_code)
+            SAVE_LINE(rom_size_line)
         END_DIRECTIVE
 
         BEGIN_DIRECTIVE(DIR_ROM_HEADER, size_t, state->header.offset, DEFAULT_HEADER_OFFSET)
@@ -401,6 +404,7 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
             }, {
                 USE_PARSER(uint8_t, size_code)
             })
+            SAVE_LINE(rom_declsize_line)
         END_DIRECTIVE
 
         END_DIRECTIVE_BLOCK
@@ -412,24 +416,20 @@ ErrorInfo* preprocess(AssemblerState *state, const LineBuffer *source)
         line = prev;
     }
 
-    // TODO: if giving rom size, check header offset is in rom size range
-    // TODO: if giving reported and actual rom size, check reported is <= actual
+    if (state->rom_size && state->header.offset + HEADER_SIZE > state->rom_size) {
+        ei = error_info_create(rom_size_line, ET_PREPROC, ED_PP_HEADER_RANGE);
+        goto cleanup;
+    }
 
-    state->rom_size = 8;  // TODO
+    if (state->rom_size && state->header.rom_size &&
+            size_code_to_bytes(state->header.rom_size) > state->rom_size) {
+        ei = error_info_create(rom_size_line, ET_PREPROC, ED_PP_DECLARE_RANGE);
+        error_info_append(ei, rom_declsize_line);
+        goto cleanup;
+    }
 
     cleanup:
     asm_lines_free(condemned);
     state->lines = dummy.next;  // Fix list head if first line was a directive
-
-#ifdef DEBUG_MODE
-    DEBUG("Dumping ASMLines:")
-    const ASMLine *temp = state->lines;
-    while (temp) {
-        DEBUG("- %-40.*s [%s:%02zu]", (int) temp->length, temp->data,
-              temp->filename, temp->original->lineno)
-        temp = temp->next;
-    }
-#endif
-
     return ei;
 }
diff --git a/src/rom.c b/src/rom.c
index 4fa0e5b..de3e07f 100644
--- a/src/rom.c
+++ b/src/rom.c
@@ -15,7 +15,6 @@
 
 #define NUM_LOCATIONS 3
 #define MAGIC_LEN 8
-#define HEADER_SIZE 16
 #define SIZE_CODE_BUF 8
 
 static size_t header_locations[NUM_LOCATIONS] = {0x7FF0, 0x3FF0, 0x1FF0};
diff --git a/src/rom.h b/src/rom.h
index 557eae1..8dde1e4 100644
--- a/src/rom.h
+++ b/src/rom.h
@@ -6,6 +6,8 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#define HEADER_SIZE 16
+
 /* Error strings */
 
 static const char* rom_err_isdir     = "Is a directory";