diff --git a/include/fh_mem.h b/include/fh_mem.h
index e7e5506..f636e98 100644
--- a/include/fh_mem.h
+++ b/include/fh_mem.h
@@ -7,6 +7,8 @@
 #ifndef FORTH_FH_MEM_H
 #define FORTH_FH_MEM_H
 
+void fh_fill_input_buffer(struct fh_thread_s *fh, const char *data, size_t num);
+
 void fh_align(struct fh_thread_s *fh);
 
 void fh_setbase(struct fh_thread_s *fh, uint32_t base);
diff --git a/include/fh_runtime.h b/include/fh_runtime.h
index f80d893..ab5ee6a 100644
--- a/include/fh_runtime.h
+++ b/include/fh_runtime.h
@@ -148,6 +148,11 @@ struct fh_thread_s {
   /** Address of the last dict word */
   uint32_t dict_last;
 
+  /** Input buffer parse position */
+  uint32_t inputptr;
+  /** Input buffer total content size */
+  uint32_t inputlen;
+
   /** Forth state */
   enum fh_state state;
 
@@ -158,6 +163,10 @@ struct fh_thread_s {
   uint32_t base;
 };
 
+#define HEAP_END      (HEAP_SIZE - WORDBUF_SIZE - INPUT_BUFFER_SIZE)
+#define WORDBUF_ADDR  HEAP_END
+#define INPUTBUF_ADDR (HEAP_END + WORDBUF_SIZE)
+
 enum fh_error fh_add_word(const struct fh_word_s *w, struct fh_thread_s *fh);
 
 void fh_setstate(struct fh_thread_s *fh, enum fh_state state, enum fh_substate substate);
diff --git a/include/forth.h b/include/forth.h
index cd3af4b..dc64cc8 100644
--- a/include/forth.h
+++ b/include/forth.h
@@ -16,6 +16,6 @@
 struct fh_thread_s;
 
 enum fh_error fh_init(struct fh_thread_s *fh);
-enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf);
+enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf, size_t len);
 
 #endif //FORTH_H
diff --git a/src/fh_builtins.c b/src/fh_builtins.c
index 88ec39d..bf7bcaf 100644
--- a/src/fh_builtins.c
+++ b/src/fh_builtins.c
@@ -992,7 +992,7 @@ static enum fh_error w_pad(struct fh_thread_s *fh, const struct fh_word_s *w)
   (void) w;
   enum fh_error rv;
   uint32_t addr = fh->here + PAD_OFFSET;
-  if (addr + 84 >= HEAP_SIZE) {
+  if (addr + 84 >= HEAP_END) {
     LOGE("Heap overflow, PAD is too small!");
     return FH_ERR_HEAP_FULL;
   }
diff --git a/src/fh_mem.c b/src/fh_mem.c
index 7cd0777..1a7818e 100644
--- a/src/fh_mem.c
+++ b/src/fh_mem.c
@@ -1,10 +1,23 @@
 #include <string.h>
+#include <assert.h>
 
 #include "fh_print.h"
 #include "fh_error.h"
 #include "fh_runtime.h"
 #include "fh_mem.h"
 
+// Important distinction: HEAP_END is the end of the normally addressable region. HEAP_SIZE is the full memory area.
+//  Buffers are placed at the end of the heap!
+
+void fh_fill_input_buffer(struct fh_thread_s *fh, const char *data, size_t num)
+{
+  assert(num <= INPUT_BUFFER_SIZE);
+  memcpy(&fh->heap[INPUTBUF_ADDR], data, num);
+  fh->heap[INPUTBUF_ADDR+num] = 0; // terminator
+  fh->inputptr = 0;
+  fh->inputlen = num;
+}
+
 void fh_align(struct fh_thread_s *fh)
 {
   fh->here = WORDALIGNED(fh->here);
@@ -102,7 +115,7 @@ enum fh_error fh_heap_reserve(
 {
   uint32_t p = fh->here;
 
-  if (p + len > HEAP_SIZE) {
+  if (p + len > HEAP_END) {
     return FH_ERR_HEAP_FULL;
   }
 
@@ -121,6 +134,11 @@ enum fh_error fh_heap_reserve(
 /** Write bytes to heap at a given location. The region must have been previously allocated! */
 void fh_heap_write(struct fh_thread_s *fh, uint32_t addr, const void *src, uint32_t len)
 {
+  if (addr > HEAP_SIZE) {
+    LOGE("Attempted write past end of heap");
+    return;
+  }
+
   memcpy(&fh->heap[addr], src, len);
 }
 
@@ -149,14 +167,14 @@ char *fh_str_at(struct fh_thread_s *fh, uint32_t addr) {
 }
 
 struct fh_instruction_s *fh_instr_at(struct fh_thread_s *fh, uint32_t addr) {
-  if (addr >= HEAP_SIZE) {
+  if (addr >= HEAP_END) {
     LOGE("fh_instr_at out of bounds!");
   }
   return (void *) &fh->heap[addr];
 }
 
 struct fh_word_s *fh_word_at(struct fh_thread_s *fh, uint32_t addr) {
-  if (addr >= HEAP_SIZE) {
+  if (addr >= HEAP_END) {
     LOGE("fh_word_at out of bounds!");
   }
   return (struct fh_word_s *) &fh->heap[addr];
diff --git a/src/fh_runtime.c b/src/fh_runtime.c
index c090523..31a9237 100644
--- a/src/fh_runtime.c
+++ b/src/fh_runtime.c
@@ -233,6 +233,7 @@ static enum fh_error fh_handle_quoted_string(
   if (fh->state == FH_STATE_INTERPRET) {
     switch (fh->substate) {
       case FH_SUBSTATE_S_QUOTE:
+        addr = fh->here;
         TRY(fh_heap_put(fh, start, len));
         TRY(ds_push(fh, addr));
         TRY(ds_push(fh, len));
@@ -461,27 +462,36 @@ static inline bool isnl(char c)
 }
 
 /** Process a line read from input */
-enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf)
+enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf, size_t len)
 {
   enum fh_error rv;
-  const char *rp = linebuf;
+
+#define ReadPtr ((char*)(&fh->heap[INPUTBUF_ADDR + fh->inputptr]))
+#define ReadPos (fh->inputptr)
+#define ReadLen (fh->inputlen)
+
+  fh_fill_input_buffer(fh, linebuf, len);
+
   char c;
 
   if (!fh_globals.interactive) {
     LOGI("%s", linebuf);
   }
 
-  while (0 != (c = *rp) && fh->state != FH_STATE_SHUTDOWN) {
+  while (ReadPos < ReadLen && fh->state != FH_STATE_SHUTDOWN) {
+    c = *ReadPtr;
     /* end on newline */
     if (isnl(c)) {
       goto done;
     }
     /* skip whitespace */
     if (isspace(c)) {
-      rp++;
+      ReadPos++;
       continue;
     }
 
+    const char * const rp = ReadPtr;
+
     char *end;
     size_t length;
     switch (fh->substate) {
@@ -520,7 +530,7 @@ enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf)
         }
 
         if (end) {
-          rp = end + 1;
+          ReadPos += length + 1;
         } else {
           goto done;
         }
@@ -534,7 +544,7 @@ enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf)
           LOG("Quoted string: \"%.*s\"", (int) length, rp);
           TRY(fh_handle_quoted_string(fh, rp, length));
           fh_setsubstate(fh, FH_SUBSTATE_NONE);
-          rp = end + 1;
+          ReadPos += length + 1;
         } else {
           /* no end. this is weird. */
           LOGE("Unterminated quoted string!");
@@ -545,9 +555,10 @@ enum fh_error fh_process_line(struct fh_thread_s *fh, const char *linebuf)
       case FH_SUBSTATE_PAREN_COMMENT:
         end = strchr(rp, ')');
         if (end) {
+          length = end - rp;
           LOG("Discard inline comment");
           fh_setsubstate(fh, FH_SUBSTATE_NONE);
-          rp = end + 1;
+          ReadPos += length + 1;
         } else {
           /* no end, discard all */
           LOGE("Unterminated parenthesis comment");
diff --git a/src/main.c b/src/main.c
index f8bd195..d411b72 100644
--- a/src/main.c
+++ b/src/main.c
@@ -68,7 +68,7 @@ int main(int argc, char *argv[])
       continue;
     }
 
-    rv = fh_process_line(&fh, linebuf);
+    rv = fh_process_line(&fh, linebuf, strlen(linebuf));
     if (rv == FH_OK) {
       FHPRINT_SVC(" ok\n");
     } else {