Add variables 'LATEST' and 'HERE'

[rrq/jonasforth.git] / main.asm

diff --git a/main.asm b/main.asm
index edcba0ae9421c712218041e64d4fc0872370f8d9..cbf8a139d6e4df42f6cfc6ab17b432b39a0efbe3 100644 (file)
--- a/main.asm
+++ b/main.asm
@@ -242,15 +242,55 @@ forth HELLO, 'HELLO'
   dq NEWLINE
   dq EXIT
 
+;; Duplicate a pair of elements.
+forth_asm PAIRDUP, '2DUP'
+  pop rbx
+  pop rax
+  push rax
+  push rbx
+  push rax
+  push rbx
+  next
+
+;; Swap the top two elements on the stack.
+forth_asm SWAP, 'SWAP'
+  pop rax
+  pop rbx
+  push rax
+  push rbx
+  next
+
+;; Remove the top element from the stack.
+forth_asm DROP, 'DROP'
+  add rsp, 8
+  next
+
 ;; The INTERPRET word reads and interprets user input. It's behavior depends on
 ;; the current STATE. It provides special handling for integers. (TODO)
 forth INTERPRET, 'INTERPRET'
+  ;; Read word
   dq READ_WORD
-  dq FIND
+  dq PAIRDUP
+  ;; Stack is (word length word length).
+  dq FIND                       ; Try to find word
+  dq DUP_
+  dq ZBRANCH, 8 * 8             ; Check if word is found
+
+  ;; Word is found, execute it
   dq TCFA
+  ;; Stack is (word length addr)
+  dq SWAP, DROP
+  dq SWAP, DROP
+  ;; Stack is (addr)
   dq EXEC
   dq EXIT
 
+  ;; No word is found, assume it is an integer literal
+  ;; Stack is (word length addr)
+  dq DROP
+  dq PARSE_NUMBER
+  dq EXIT
+
 ;; .U prints the value on the stack as an unsigned integer in hexadecimal.
 forth_asm DOTU, '.U'
   mov [.length], 0
@@ -332,6 +372,19 @@ forth STATE, 'STATE'
   dq LIT, var_STATE
   dq EXIT
 
+;; Get the location of the LATEST variable. It can be set with '!' and read with
+;; '@'.
+forth LATEST, 'LATEST'
+  dq LIT, latest_entry
+  dq EXIT
+
+;; Get the location at which compiled words are expected to be added. This
+;; pointer is usually modified automatically when calling ',', but we can also
+;; read it manually with 'HERE'.
+forth HERE, 'HERE'
+  dq LIT, here
+  dq EXIT
+
 forth MAIN, 'MAIN'
   dq HELLO
   dq INTERPRET
@@ -340,6 +393,9 @@ forth MAIN, 'MAIN'
 
 segment readable writable
 
+;; The LATEST variable holds a pointer to the word that was last added to the
+;; dictionary. This pointer is updated as new words are added, and its value is
+;; used by FIND to look up words.
 latest_entry dq initial_latest_entry
 
 ;; The STATE variable is 0 when the interpreter is executing, and non-zero when
@@ -357,6 +413,10 @@ DOTU.rbuffer rq 16
 DOTU.length dq ?
 DOTU.printed_length dq ?
 
+;; Reserve space for compiled words, accessed through HERE.
+here dq here_top
+here_top rq $2000
+
 ;; Return stack
 rq $2000
 return_stack_top: