cvm: improve comments

They were a bit terse.

cvm/vm.c

@@ -21,6 +21,8 @@ static VM vm;
 static uint64_t blkop = 0; // 5 bytes
 static FILE *blkfp;
 
+// Read single byte from I/O handler, if set. addr is a word only because of
+// Forth's cell size, but can't actually address more than a byte-ful of ports.
 static byte io_read(word addr)
 {
     addr &= 0xff;
@@ -44,6 +46,9 @@ static void io_write(word addr, byte val)
     }
 }
 
+// I/O hook to read/write a chunk of 1024 byte to blkfs at specified blkid.
+// This is used by EFS@ and EFS! in xcomp.fs.
+// See comment above BLK_PORT define for poking convention.
 static void iowr_blk(byte val)
 {
     blkop <<= 8;
@@ -62,29 +67,37 @@ static void iowr_blk(byte val)
     }
 }
 
+// get/set word from/to memory
 static word gw(word addr) { return vm.mem[addr+1] << 8 | vm.mem[addr]; }
 static void sw(word addr, word val) {
     vm.mem[addr] = val;
     vm.mem[addr+1] = val >> 8;
 }
+// pop word from SP
 static word pop() {
     if (vm.uflw) return 0;
     if (vm.SP >= SP_ADDR) { vm.uflw = true; }
     return vm.mem[vm.SP++] | vm.mem[vm.SP++] << 8;
 }
+// push word to SP
 static void push(word x) {
     vm.SP -= 2; sw(vm.SP, x);
     if (vm.SP < vm.minSP) { vm.minSP = vm.SP; }
 }
+// pop word from RS
 static word popRS() {
     if (vm.uflw) return 0;
     if (vm.RS <= RS_ADDR) { vm.uflw = true; }
     word x = gw(vm.RS); vm.RS -= 2; return x;
 }
+// push word to RS
 static void pushRS(word val) {
     vm.RS += 2; sw(vm.RS, val);
     if (vm.RS > vm.maxRS) { vm.maxRS = vm.RS; }
 }
+
+// The functions below directly map to native forth words defined in the
+// dictionary (B30)
 static void execute(word wordref) {
     byte wtype = vm.mem[wordref];
     if (wtype == 0) { // native
@@ -242,6 +255,7 @@ static void MINUS2() { push(pop()-2); }
 static void PLUS2() { push(pop()+2); }
 static void RSHIFT() { word u = pop(); push(pop()>>u); }
 static void LSHIFT() { word u = pop(); push(pop()<<u); }
+
 static void native(NativeWord func) {
     vm.nativew[vm.nativew_count++] = func;
 }
@@ -284,6 +298,7 @@ VM* VM_init() {
         vm.iowr[i] = NULL;
     }
     vm.iowr[BLK_PORT] = iowr_blk;
+    // Added in the same order as in xcomp.fs
     native(EXIT);
     native(_br_);
     native(_cbr_);

cvm/vm.h

@@ -14,28 +14,30 @@ typedef void (*NativeWord) ();
 typedef byte (*IORD) ();
 typedef void (*IOWR) (byte data);
 
-/* Native word placement
-    Being a C VM, all actual native code is outside the VM's memory. However,
-    we have a stable ABI to conform to. VM_init() configures the memory by
-    placing references to stable words at proper offsets, and then add all other
-    native words next to it. This will result in a "boot binary" that is much
-    more compact than a real Collapse OS memory layout.
-*/
 typedef struct {
     byte mem[0x10000];
-    word SP;
-    word RS;
-    word IP;
+    word SP; // parameter Stack Pointer
+    word RS; // Return Stack pointer
+    word IP; // Interpreter Pointer
+    // A list of native words' code. This is filled in VM_init() by calls to
+    // native(). The order is very important because we refer to these elements
+    // by index. For example, "0x42 CODE FOO" in Forth creates the native word
+    // FOO which, when executed, will call the code at index 0x42 in this array.
     NativeWord nativew[0x100];
     byte nativew_count;
     // Array of 0x100 function pointers to IO read and write routines. Leave to
     // NULL when IO port is unhandled.
     IORD iord[0x100];
     IOWR iowr[0x100];
-    word xcurrent; // only used during native bootstrap
+    // Used for keeping track of max RS and min SP during the lifetime of the
+    // program. Useful for debugging.
     word maxRS;
     word minSP;
     bool running;
+    // Whether we're in stack underflow situation. Alters the behavior of some
+    // core action, notably popping. Doesn't stay set for more than a single
+    // execute cycle. The goal is to avoid over-popping in native words that
+    // pop more than once and thus corrupt memory.
     bool uflw;
 } VM;