From e8e4b5dd2475943c7a4500b1af3d67f7b4eae414 Mon Sep 17 00:00:00 2001
From: slederer <dev@insignificance.de>
Date: Sun, 8 Feb 2026 01:58:50 +0100
Subject: [PATCH] tridoraemu,docs: emulate vgafb shifter/maskgen and new iomem
 layout

---
 doc/vga.md                | 20 +++++------
 tridoraemu/cpu.go         |  1 +
 tridoraemu/framebuffer.go | 72 ++++++++++++++++++++++++++++++++++++---
 3 files changed, 78 insertions(+), 15 deletions(-)

diff --git a/doc/vga.md b/doc/vga.md
index 76520f2..3ef8f8d 100644
--- a/doc/vga.md
+++ b/doc/vga.md
@@ -9,11 +9,11 @@ Registers
 | _FB_PS_ | $90C | Palette Select |
 | _FB_PD_ | $910 | Palette Data |
 | _FB_CTL_ | $914 | Control Register |
-| _FB_SHIFTER | $918 | Shift Assist Register |
-| _FB_SHIFTCOUNT | $91C | Shift Count Register |
-| _FB_SHIFTERM   | $920 | Shifted Mask Register |
-| _FB_SHIFTERSP  | $924 | Shifter Spill Register |
-| _FB_MASKGEN    | $928 | Mask Generator Register |
+| _FB_SHIFTER_ | $918 | Shift Assist Register |
+| _FB_SHIFTCOUNT_ | $91C | Shift Count Register |
+| _FB_SHIFTERM_   | $920 | Shifted Mask Register |
+| _FB_SHIFTERSP_  | $924 | Shifter Spill Register |
+| _FB_MASKGEN_    | $928 | Mask Generator Register |
 
 ## Pixel Data
 Pixel data is organized in 32-bit-words. With four bits per pixel, one word
@@ -121,12 +121,12 @@ For each four bits of a pixel, the corresponding four mask bits
 are all set to one if the pixel value is not zero.
 
 This can be used to combine foreground and background pixel data
-with a pixel value of zero for a transparent background color.
+where a pixel value of zero is used to indicate a transparent foreground pixel.
 
-Usually, the mask will be inverted with a *NOT* instruction
-to clear all pixels in the background that are set in the foreground
-with an *AND* instruction
-before *ORing* foreground and background together.
+Usually, the mask will be inverted with a *NOT* instruction.
+The result can then be used to clear all pixels in the background
+that are set in the foreground, using an *AND* instruction.
+As the last step, foreground and masked background data can be combined with an *OR* instruction.
 
 Example in hexadecimal, each digit is a pixel:
 | Pixel Data | Mask |
diff --git a/tridoraemu/cpu.go b/tridoraemu/cpu.go
index 9d6b08d..f1ae4de 100644
--- a/tridoraemu/cpu.go
+++ b/tridoraemu/cpu.go
@@ -249,6 +249,7 @@ func (c *CPU) step() error {
 		var name string
 		if (insWord & 1) == 1 {
 			name = "STORE.B"
+			operand &= ^1
 			ea = c.BP + word(operand)
 		} else {
 			name = "STORE"
diff --git a/tridoraemu/framebuffer.go b/tridoraemu/framebuffer.go
index 189100a..71776ca 100644
--- a/tridoraemu/framebuffer.go
+++ b/tridoraemu/framebuffer.go
@@ -10,11 +10,16 @@ import (
 const VmemWords = 32768
 const PaletteSlots = 16
 const FB_RA = 0
-const FB_WA = 1
-const FB_IO = 2
-const FB_PS = 3
-const FB_PD = 4
-const FB_CTL= 5
+const FB_WA = 4
+const FB_IO = 8
+const FB_PS = 12
+const FB_PD = 16
+const FB_CTL= 20
+const FB_SHIFTER = 24
+const FB_SHIFTCOUNT = 28
+const FB_SHIFTERM = 32
+const FB_SHIFTERSP = 36
+const FB_MASKGEN = 40
 
 const PixelMask = 0b11110000000000000000000000000000
 const PixelPerWord = 8
@@ -33,6 +38,9 @@ type Framebuffer struct {
 	vmem [VmemWords]word
 	readCount int
         paletteChanged bool
+	shiftAssistData word
+	shiftAssistCount int
+	maskGenData word
 }
 
 func (f *Framebuffer) initialize() {
@@ -53,6 +61,11 @@ func (f *Framebuffer) read(byteaddr word) (word, error) {
 	case FB_PS: result = f.paletteSlot
 	case FB_PD: result = f.readPalette()
 	case FB_CTL: result = f.readCtl()
+	case FB_SHIFTER: result = f.readShiftAssist()
+	case FB_SHIFTCOUNT: result = 0xFFFFFFF
+	case FB_SHIFTERM: result = f.readShifterM()
+	case FB_SHIFTERSP: result = f.readShifterSp()
+	case FB_MASKGEN: result = f.readMaskGen()
 	default:
 	}
 	return result, nil
@@ -67,6 +80,11 @@ func (f *Framebuffer) write(value word, byteaddr word) (error) {
 	case FB_PS: f.paletteSlot = value
 	case FB_PD: f.writePalette(value)
 	case FB_CTL: f.writeCtl(value)
+	case FB_SHIFTER: f.writeShiftAssist(value)
+	case FB_SHIFTCOUNT: f.writeShiftCount(value)
+	case FB_SHIFTERM:
+	case FB_SHIFTERSP:
+	case FB_MASKGEN: f.writeMaskGen(value)
 	default:
 	}
 
@@ -152,3 +170,47 @@ func (f *Framebuffer) readCtl() word {
 
 func (f *Framebuffer) writeCtl(value word) {
 }
+
+func (f *Framebuffer) writeShiftAssist(value word) {
+	f.shiftAssistData = value
+	f.shiftAssistCount = 0
+}
+
+func (f *Framebuffer) readShiftAssist() word {
+	return f.shiftAssistData >> (f.shiftAssistCount * 4)
+}
+
+func (f *Framebuffer) writeShiftCount(value word) {
+	f.shiftAssistCount = int(value & 0x7)
+}
+
+func (f *Framebuffer) readShifterM() word {
+	return convertToMask(f.readShiftAssist())
+}
+
+func pixelToMask(pixels word, mask word) word {
+	if (pixels & mask) != 0 { return  mask } else { return 0 }
+}
+
+func convertToMask(pixels word) word {
+	return  pixelToMask(pixels, 0xF0000000) |
+		pixelToMask(pixels, 0x0F000000) |
+		pixelToMask(pixels, 0x00F00000) |
+		pixelToMask(pixels, 0x000F0000) |
+		pixelToMask(pixels, 0x0000F000) |
+		pixelToMask(pixels, 0x00000F00) |
+		pixelToMask(pixels, 0x000000F0) |
+		pixelToMask(pixels, 0x0000000F)
+}
+
+func (f *Framebuffer) readShifterSp() word {
+	return word(f.shiftAssistData << ((8-f.shiftAssistCount)*4))
+}
+
+func (f *Framebuffer) writeMaskGen(value word) {
+	f.maskGenData = value
+}
+
+func (f *Framebuffer) readMaskGen() word {
+	return convertToMask(f.maskGenData)
+}