From 0194f9c3bbb460647ce5a22e4e8e7859398d8da0 Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Mon, 8 May 2023 22:52:26 +0100
Subject: Support read from the screen

---
 tr8vm.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

(limited to 'tr8vm.c')

diff --git a/tr8vm.c b/tr8vm.c
index 59d08b7..69a0123 100644
--- a/tr8vm.c
+++ b/tr8vm.c
@@ -121,24 +121,41 @@ uint8_t port(uint8_t p, uint8_t v)
 {
     switch (p)
     {
+        /* blitter control */
         case 0xb0:
+            /* settings mode */
             if (v & 128)
             {
                 blt_set = 1;
                 blt_paramc = 0;
                 return p;
             }
-            if (v & 1)
+            /* draw (read or write) */
+            if (v & 5)
             {
                 blt_set = 0;
 
+                /* can't be both */
+                if ((v & 1) && (v & 4))
+                    return 0xff;
+
+                /* missing parameters */
                 if (blt_paramc != 6)
                     return 0xff;
 
-                /* draw */
+                /* 1: write into vram */
                 uint16_t src = blt_param[0] | (blt_param[1] << 8);
                 uint16_t dst = VIDEO_RAM + blt_param[2] + blt_param[3] * 128;
 
+                /* 4: read from vram */
+                if (v & 4)
+                {
+                    /* swap */
+                    uint16_t t = dst;
+                    dst = src;
+                    src = t;
+                }
+
                 for (uint8_t y = 0; y < blt_param[5]; y++)
                     for (uint8_t x = 0; x < blt_param[4]; x++)
                     {
@@ -155,6 +172,7 @@ uint8_t port(uint8_t p, uint8_t v)
             }
             return 0xff;
 
+        /* blitter settings */
         case 0xb1:
             if (!blt_set || blt_paramc > 5)
                 return 0xff;
-- 
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