[9front] games/gb: various HDMA fixes

[Michael Forney <mforney@mforney.org>]

H-blank DMA should only transfer 16 bytes per h-blank, rather than
waiting for the first h-blank and then transferring the whole size.

HDMAC should read 0xff when the transfer is finished, and 0 in the
high bit when the transfer is ongoing. Also, if 0 is written in the
high bit, the current transfer should be aborted.

Introduce two flags, DMAREADY and DMAHBLANK rather than special
constants 1 and -1. If dma is non-zero, there is an ongoing DMA. If
DMAREADY is set, the next chunk is ready to transfer.

Reference: https://gbdev.io/pandocs/#ff55-hdma5-cgb-mode-only-new-dma-length-mode-start

diff 9f73daae4c186334de5c9ee5b6bb0d9aa598f9bd 3fc115a6ff6eef65210fb78a51128958d2b55910
--- a/sys/src/games/gb/dat.h	Sat Feb  6 04:50:06 2021
+++ b/sys/src/games/gb/dat.h	Sun Feb  7 15:22:39 2021
@@ -8,7 +8,7 @@
 extern uchar vram[16384];
 extern int nrom, nback, nbackbank;
 extern u32int pal[64];
-extern s8int dma;
+extern u8int dma;
 extern u32int divclock;
 
 extern Event *elist;
@@ -113,6 +113,9 @@
 	FEATRAM = 1,
 	FEATBAT = 2,
 	FEATTIM = 4,
+
+	DMAREADY = 1,
+	DMAHBLANK = 2,
 	
 	INIT = -1,
 	SAVE = -2,
--- a/sys/src/games/gb/gb.c	Sat Feb  6 04:50:06 2021
+++ b/sys/src/games/gb/gb.c	Sun Feb  7 15:22:39 2021
@@ -308,7 +308,7 @@
 			qlock(&pauselock);
 			qunlock(&pauselock);
 		}
-		if(dma > 0)
+		if(dma & DMAREADY)
 			t = dmastep();
 		else
 			t = step();
--- a/sys/src/games/gb/mem.c	Sat Feb  6 04:50:06 2021
+++ b/sys/src/games/gb/mem.c	Sun Feb  7 15:22:39 2021
@@ -15,7 +15,7 @@
 u32int divclock;
 int prish;
 MBC3Timer timer, timerl;
-s8int dma;
+u8int dma;
 u32int white;
 u32int moncols[4];
 
@@ -175,7 +175,14 @@
 	case HDMAC:
 		if((mode & COL) == 0)
 			goto ff;
-		dma = (v & 0x80) != 0 ? -1 : 1;
+		if(v & 0x80){
+			v &= 0x7f;
+			dma = DMAHBLANK;
+		}else if(dma){
+			v |= 0x80;
+			dma = 0;
+		}else
+			dma = DMAREADY;
 		break;
 	case NR10: v |= 0x80; goto snd;
 	case NR14: case NR24: v |= 0x38; goto snd;
@@ -534,6 +541,7 @@
 	reg[VBK] = 0xfe;
 	reg[SVBK] = 0xf8;
 	reg[IF] = 0xe0;
+	reg[HDMAC] = 0xff;
 }
 
 void
@@ -572,7 +580,7 @@
 {
 	int i;
 	u16int sa, da;
-	
+
 	sa = (reg[HDMASL] | reg[HDMASH] << 8) & 0xfff0;
 	da = (reg[HDMADL] | reg[HDMADH] << 8) & 0x1ff0 | 0x8000;
 	for(i = 0; i < 16; i++)
@@ -583,12 +591,9 @@
 	reg[HDMADL] += 16;
 	if((reg[HDMADL] & 0xf0) == 0)
 		reg[HDMADH]++;
-	if((reg[HDMAC] & 0x7f) == 0)
+	if(--reg[HDMAC] == 0xff)
 		dma = 0;
-	else{
-		reg[HDMAC]--;
-		if((reg[HDMAC] & 0x80) != 0)
-			dma = 1;
-	}
+	else if(dma & DMAHBLANK)
+		dma &= ~DMAREADY;
 	return 64;
 }
--- a/sys/src/games/gb/ppu.c	Sat Feb  6 04:50:06 2021
+++ b/sys/src/games/gb/ppu.c	Sun Feb  7 15:22:39 2021
@@ -338,8 +338,8 @@
 			reg[IF] |= IRQLCDS;
 		t = hblclock + 456 * 2 - clock;
 		addevent(&evhblank, t < 0 ? 456 * 2 : t);
-		if(dma < 0)
-			dma = 1;
+		if(dma & DMAHBLANK)
+			dma |= DMAREADY;
 		break;
 	}
 }