Allow $ in equ statements, which evaluates to the current address

Also allow $+num, $-num, $*num, $/num, and $%num
This will perform basic evaluation of address and number

source/a80/i80.d

@@ -1165,10 +1165,15 @@ static void cpi()
  */
 static void equ()
 {
+ ushort a;
+
  if (lab.empty)
  err("must have a label in equ statement");
 
- auto a = numcheck();
+ if (a1[0] == '$')
+ a = dollar();
+ else
+ a = numcheck(a1);
 
  if (pass == 1)
  addsym(lab, a);
@@ -1181,7 +1186,7 @@ static void db()
 {
  argcheck(!a1.empty && a2.empty);
  if (isDigit(a1[0])) {
- auto a = numcheck();
+ auto a = numcheck(a1);
  passAct(1, a);
  } else {
  if (pass == 1) {
@@ -1222,12 +1227,12 @@ static void ds()
  if (!lab.empty)
  addsym(lab, addr);
  } else {
- auto a = numcheck();
+ auto a = numcheck(a1);
  for (size_t i = 0; i < a; i++)
  output ~= cast(ubyte)0;
  }
 
- addr += numcheck();
+ addr += numcheck(a1);
 }
 
 /**
@@ -1238,7 +1243,7 @@ static void org()
  argcheck(lab.empty && !a1.empty && a2.empty);
  if (isDigit(a1[0])) {
  if (pass == 1)
- addr = numcheck();
+ addr = numcheck(a1);
  } else {
  err("org must take a number");
  }
@@ -1345,10 +1350,7 @@ static void imm(int type)
  check = a1;
 
  if (isDigit(check[0])) {
- if (check[check.length - 1] == 'h')
- dig = to!ushort(chop(check), 16);
- else
- dig = to!ushort(check, 10);
+ dig = numcheck(check);
  } else {
  for (size_t i = 0; i < stab.length; i++) {
  if (check == stab[i].name) {
@@ -1379,7 +1381,7 @@ static void a16()
  bool found = false;
 
  if (isDigit(a1[0])) {
- dig = numcheck();
+ dig = numcheck(a1);
  } else {
  for (size_t i = 0; i < stab.length; i++) {
  if (a1 == stab[i].name) {
@@ -1421,14 +1423,41 @@ static void argcheck(bool passed)
 /**
  * Check if a number is decimal or hex.
  */
-static ushort numcheck()
+static ushort numcheck(string input)
 {
  ushort num;
 
- if (a1[a1.length - 1] == 'h')
- num = to!ushort(chop(a1), 16);
+ if (input[input.length - 1] == 'h')
+ num = to!ushort(chop(input), 16);
  else
- num = to!ushort(a1, 10);
+ num = to!ushort(input, 10);
+
+ return num;
+}
+
+/**
+ * If the argument to EQU begins with $, we need to parse that.
+ * Our syntax differs a little from the CP/M assembler.
+ * And it only deals with simple expressions.
+ */
+static ushort dollar()
+{
+ ushort num = addr;
+
+ if (a1.length > 1) {
+ if (a1[1] == '+')
+ num += numcheck(a1[2..$]);
+ else if (a1[1] == '-')
+ num -= numcheck(a1[2..$]);
+ else if (a1[1] == '*')
+ num *= numcheck(a1[2..$]);
+ else if (a1[1] == '/')
+ num /= numcheck(a1[2..$]);
+ else if (a1[1] == '%')
+ num %= numcheck(a1[2..$]);
+ else
+ err("invalid operator in equ");
+ }
 
  return num;
 }