Add word "BLK!"

as well as emulator support for it. We can now write an edited
block back to "blkfs".

emul/forth/forth.c

@@ -12,8 +12,9 @@
 // failing, send a non-zero value to RET_PORT to indicate failure
 #define RET_PORT 0x01
 // Port for block reads. Write 2 bytes, MSB first, on that port and then
-// read 1024 bytes from the same port.
+// read 1024 bytes from the DATA port.
 #define BLK_PORT 0x03
+#define BLKDATA_PORT 0x04
 
 static int running;
 static FILE *fp;
@@ -44,7 +45,16 @@ static void iowr_ret(uint8_t val)
     retcode = val;
 }
 
-static uint8_t iord_blk()
+static void iowr_blk(uint8_t val)
+{
+    blkid <<= 8;
+    blkid |= val;
+    if (blkfp != NULL) {
+        fseek(blkfp, blkid*1024, SEEK_SET);
+    }
+}
+
+static uint8_t iord_blkdata()
 {
     uint8_t res = 0;
     if (blkfp != NULL) {
@@ -56,12 +66,10 @@ static uint8_t iord_blk()
     return res;
 }
 
-static void iowr_blk(uint8_t val)
+static void iowr_blkdata(uint8_t val)
 {
-    blkid <<= 8;
-    blkid |= val;
     if (blkfp != NULL) {
-        fseek(blkfp, blkid*1024, SEEK_SET);
+        putc(val, blkfp);
     }
 }
 
@@ -99,8 +107,9 @@ int main(int argc, char *argv[])
     m->iord[STDIO_PORT] = iord_stdio;
     m->iowr[STDIO_PORT] = iowr_stdio;
     m->iowr[RET_PORT] = iowr_ret;
-    m->iord[BLK_PORT] = iord_blk;
     m->iowr[BLK_PORT] = iowr_blk;
+    m->iord[BLKDATA_PORT] = iord_blkdata;
+    m->iowr[BLKDATA_PORT] = iowr_blkdata;
     // initialize memory
     for (int i=0; i<sizeof(KERNEL); i++) {
         m->mem[i] = KERNEL[i];

emul/forth/run.fs

@@ -1,14 +1,22 @@
 : EFS@
     256 /MOD 3 PC! 3 PC!
     1024 0 DO
-        3 PC@
+        4 PC@
         BLK( I + C!
     LOOP
 ;
+: EFS!
+    256 /MOD 3 PC! 3 PC!
+    1024 0 DO
+        BLK( I + C@ 4 PC!
+    LOOP
+;
+
 : INIT
     CURRENT @ HERE !
     BLK$
     ['] EFS@ BLK@* !
+    ['] EFS! BLK!* !
     RDLN$
     Z80A$
     LIT< _sys [entry]

forth/blk.fs

@@ -19,11 +19,15 @@
     -1 BLK> !
 ;
 
+( n -- )
 : BLK@
     DUP BLK> @ = IF DROP EXIT THEN
     DUP BLK> ! BLK@* @ EXECUTE
 ;
 
+( -- )
+: BLK! BLK> @ BLK!* @ EXECUTE ;
+
 : .2 DUP 10 < IF SPC THEN . ;
 
 : LIST