Support read from the screen

2 files changed, 27 insertions, 6 deletions

diff --git a/README.md b/README.md
index 4f70bf0..f8eb7e5 100644
--- a/README.md
+++ b/README.md
@@ -77,8 +77,10 @@ On a successful port call, the read value equals to the port number, so the call
 To draw with the blitter the following steps are required:
 
 1. put the blitter in settings mode using control port `0xb0`
-2. provide the source address, and destination coordinates (x, y, width and height) using settings port `0xb1`
-3. set the blitter in one of the write modes using the control port `0xb0`
+2. provide an address, and screen coordinates (x, y, width and height) using settings port `0xb1`
+3. set the blitter in one of the draw modes using the control port `0xb0`
+
+The address will be the source data in write mode, and the destination in read mode.
 
 Step 3 can be repeated as many times as needed, the blitter will keep the settings until the control port is set in settings mode.
 
@@ -87,9 +89,10 @@ The settings byte is as follow:
 | Bits | Effect
 | --- | --- |
 | `x0000000` | Set blitter in settings mode |
-| `0000000x` | Write |
+| `0000000x` | Write to video RAM |
 | `000000x0` | The source includes a transparent bit |
-| `0xxxxx00` | Unused |
+| `00000x00` | Read from video RAM |
+| `0xxxx000` | Unused |
 
 Example:
 
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;