From 57fd14b0b3b97ef623e4503ba3f9580aac6812ab Mon Sep 17 00:00:00 2001
From: Virgil Dupras <hsoft@hardcoded.net>
Date: Mon, 16 Nov 2020 13:10:04 -0500
Subject: [PATCH] sms: fix cursor mis-display in text mode

In CURSOR!, I was using a write commande to read from VRAM and the
emulator didn't properly behave and did as if everything was fine.

The result on a real SMS was that the cursor would contain the
inverted glyph of the contents of the *old* cursor position.
---
 blk.fs             | 11 ++++++-----
 emul/z80/tms9918.c | 11 ++++++-----
 emul/z80/tms9918.h |  1 +
 3 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/blk.fs b/blk.fs
index da50eba..8debdaa 100644
--- a/blk.fs
+++ b/blk.fs
@@ -2769,7 +2769,8 @@ CODE TICKS 1 chkPS, ( n=100us )
 ( ----- 470 )
 ( Z80 driver for TMS9918. Implements grid protocol. Requires
 TMS_CTLPORT, TMS_DATAPORT and ~FNT from the Font compiler at
-B520. Load range B470-472 )
+B520. Patterns are at addr 0x0000, Names are at 0x3800.
+Load range B470-472 )
 CODE _ctl ( a -- sends LSB then MSB )
     HL POP, chkPS,
     A L LDrr, TMS_CTLPORT OUTiA,
@@ -2795,10 +2796,10 @@ them.  We insert a blank one at the end of those 7. )
     0x20 - ( glyph ) 0x5e MOD _data ;
 ( ----- 472 )
 : CURSOR! ( new old -- )
-    0x7800 OR DUP _ctl [ TMS_DATAPORT LITN ] PC@
-        0x7f AND ( new cmd glyph ) SWAP _ctl _data
-    0x7800 OR DUP _ctl [ TMS_DATAPORT LITN ] PC@
-        0x80 OR ( cmd glyph ) SWAP _ctl _data ;
+    DUP 0x3800 OR _ctl [ TMS_DATAPORT LITN ] PC@
+    0x7f AND ( new old glyph ) SWAP 0x7800 OR _ctl _data
+    DUP 0x3800 OR _ctl [ TMS_DATAPORT LITN ] PC@
+    0x80 OR ( new glyph ) SWAP 0x7800 OR _ctl _data ;
 : COLS 40 ; : LINES 24 ;
 : TMS$
     0x8100 _ctl ( blank screen )
diff --git a/emul/z80/tms9918.c b/emul/z80/tms9918.c
index a2a9d57..462fca7 100644
--- a/emul/z80/tms9918.c
+++ b/emul/z80/tms9918.c
@@ -27,6 +27,7 @@ void tms_init(TMS9918 *tms)
     memset(tms->regs, 0, 0x10);
     tms->has_cmdlsb = false;
     tms->curaddr = 0;
+    tms->databuf = 0;
     tms->width = 40*6;
     tms->height = 24*8;
 }
@@ -50,20 +51,20 @@ void tms_cmd_wr(TMS9918 *tms, uint8_t val)
     } else {
         // VRAM
         tms->curaddr = ((val&0x3f) << 8) + tms->cmdlsb;
+        if ((val & 0x40) == 0) { // reading VRAM
+            tms->databuf = tms->vram[tms->curaddr];
+        }
     }
 }
 
 uint8_t tms_data_rd(TMS9918 *tms)
 {
-    if (tms->curaddr < TMS_VRAM_SIZE) {
-        return tms->vram[tms->curaddr++];
-    } else {
-        return 0;
-    }
+    return tms->databuf;
 }
 
 void tms_data_wr(TMS9918 *tms, uint8_t val)
 {
+    tms->databuf = val;
     if (tms->curaddr < TMS_VRAM_SIZE) {
         tms->vram[tms->curaddr++] = val;
     }
diff --git a/emul/z80/tms9918.h b/emul/z80/tms9918.h
index db4b8d2..0ab2d33 100644
--- a/emul/z80/tms9918.h
+++ b/emul/z80/tms9918.h
@@ -13,6 +13,7 @@ typedef struct {
     uint8_t cmdlsb;
     bool has_cmdlsb;
     uint16_t curaddr;
+    uint8_t databuf;
     uint16_t width; // in pixels
     uint16_t height; // in pixels
 } TMS9918;