From a9c4660e11e7ec9a48259e0c64759d387f387f37 Mon Sep 17 00:00:00 2001
From: tA <s@p7.co.nz>
Date: Wed, 10 Jun 2020 22:36:58 +1200
Subject: [PATCH] added conditional jump

---
 README.md |  5 +++--
 vm.rkt    | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 54225c7..524f1f2 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ in the racket repl (comments for reader comprehension, remove before running):
      #xF0                ;               (non-instruction, will be arg for previous byte)
      #x2E #x68           ; LD L, $68
      #x77                ; LD (HL), A
-     #xC3 #x64 #x01      ; JP $0164
+     #xC2 #x64 #x01      ; JP $0164
      #x10                ; STOP
      #x7D                ; LD A, L       (address $0164)
      #x10                ; STOP
@@ -33,12 +33,13 @@ HL: $0168, AF: $6800
 $0150 > $3e $69 $26 $01 $2e $67 $77 $2e $5b $36 $e6 $e6 $f0 $2e $68 $77 $c3 $64 $01 $10 $7d $10 $00 $69 $60 < $0168
 ```
 
-here we can see the program loaded starting from address `$0150`, including the `XOR` instruction loaded to `$015B` during execution, and the `$69` we loaded to memory, along with the result of `XOR $F0` we loaded to memory, at `$0167` and `$0168` respectively. we can also see that the `A` register is `$68`, the same as the `L` register, as the `JP` instruction executed properly and we skipped the `STOP` at address `$0163`.
+here we can see the program loaded starting from address `$0150`, including the `XOR` instruction loaded to `$015B` during execution, and the `$69` we loaded to memory, along with the result of `XOR $F0` we loaded to memory, at `$0167` and `$0168` respectively. we can also see that the `A` register is `$68`, the same as the `L` register, as the conditional `JP NZ` instruction executed properly and we skipped the `STOP` at address `$0163`.
 
 ## notes
 
 currently supported instructions are:
    * `JP #addr`
+   * `JP [cc], #addr`
    * `LD [reg], #imm`
    * `LD [reg], [reg]`
    * `XOR #imm`
diff --git a/vm.rkt b/vm.rkt
index 507b1c3..1dd27b9 100644
--- a/vm.rkt
+++ b/vm.rkt
@@ -14,6 +14,12 @@
 (define (make-regs)
    (make-vector 8 0))
 
+(define (get-zero mem)
+   (bitwise-bit-set? (mem-flags mem) 7))
+
+(define (get-carry mem)
+   (bitwise-bit-set? (mem-flags mem) 4))
+
 (define (get-reg x mem)
    (if (and (< x 8) (>= x 0))
        (let ([regs (mem-regs mem)])
@@ -108,8 +114,17 @@
             [y (get-byte pc m)]
             [npc (add1pc pc)]
             [x (get-byte npc m)]
-            [addr (make-16b-addr x y)])
-     (cons (jp cc addr) (set-pc npc m))))
+            [addr (make-16b-addr x y)]
+            [cs (cc-tab cc)])
+     (cons (jp cs addr) (set-pc npc m))))
+
+(define (cc-tab cc)
+   (case cc
+      [(0) 'NZ]
+      [(1) 'Z]
+      [(2) 'NC]
+      [(3) 'C]
+      [else 'uncond]))
 
 (define (make-8b-ld-reg-reg y z m)
   (cons (8b-ld-reg-reg y z)
@@ -134,7 +149,7 @@
 (define (jp cc addr)
    (case cc
       [(uncond) (jp-uncond addr)]
-      [else (nop)]))
+      [else (jp-cond cc addr)]))
 
 (define (8b-ld-reg-imm reg imm)
    (lambda (m)
@@ -146,6 +161,16 @@
                (get-reg src m)
                m)))
 
+(define (jp-cond cc addr)
+   (lambda (m)
+      (let ([c (get-carry m)]
+            [z (get-zero m)])
+         (case cc
+            [(NZ) (if (not z) (set-pc addr m) m)]
+            [(Z) (if z (set-pc addr) m)]
+            [(NC) (if (not c) (set-pc addr) m)]
+            [(C) (if c (set-pc addr) m)]))))
+
 (define (jp-uncond addr)
    (lambda (m)
       (set-pc addr m)))
@@ -245,6 +270,8 @@
          [(= #xC3 op) (make-jp 'uncond (set-pc npc m))]
          [(= 1 x)
             (make-8b-ld-reg-reg y z (set-pc npc m))]
+         [(and (= 3 x) (= z 2) (within y 0 3))
+            (make-jp y (set-pc npc m))]
          [(and (= 3 x) (within z 4 6))
             (make-alu-imm y (set-pc npc m))]
          [(and (= 0 x) (= 6 z))