From 847dba8f0745b49bcf2df567c6a759d84bc3327c Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 21 Mar 2024 11:03:28 -0400
Subject: [PATCH] Divide input pixel rate.
---
Machines/Acorn/Archimedes/Video.hpp | 30 +++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)
diff --git a/Machines/Acorn/Archimedes/Video.hpp b/Machines/Acorn/Archimedes/Video.hpp
index d1d56365d..c7d1d59c1 100644
--- a/Machines/Acorn/Archimedes/Video.hpp
+++ b/Machines/Acorn/Archimedes/Video.hpp
@@ -108,6 +108,18 @@ struct Video {
case 0xe0:
logger.error().append("TODO: video control: %08x", value);
+
+ // Set pixel rate.
+ //
+ // TODO: possibly do this as a multiplier on position counts and a divider on pixels,
+ // to maintain a consistent CRT data clock?
+ switch(value & 0b11) {
+ case 0b00: clock_divider_ = 12; break; // 4Mhz count.
+ case 0b01: clock_divider_ = 8; break; // 6Mhz count.
+ case 0b10: clock_divider_ = 6; break; // 8Mhz count.
+ case 0b11: clock_divider_ = 4; break; // i.e. 48/4 = 12Mhz position counting clock.
+ }
+ sub_clock_ = 0;
break;
//
@@ -123,7 +135,6 @@ struct Video {
sound_.set_frequency(value & 0x7f);
break;
-
default:
logger.error().append("TODO: unrecognised VIDC write of %08x", value);
break;
@@ -131,6 +142,18 @@ struct Video {
}
void tick() {
+ // Apply clock divider to get 8, 12, 16 or 24 Mhz pixel rate as user-selected,
+ // which since all event positioning is at two-pixel boundaries means a
+ // 4, 6, 8 or 12 Mhz counting clock.
+ //
+ // tick() is called at 12Mhz.
+ sub_clock_ += 4; // Count at 48 Mhz.
+ if(sub_clock_ < clock_divider_) {
+ return;
+ }
+ sub_clock_ -= clock_divider_;
+
+ // TODO: real output. For now, just count up to complete frames and pretend a retrace goes there.
++position_;
if(position_ >= horizontal_.period * vertical_.period) {
entered_sync_ = true;
@@ -152,7 +175,7 @@ private:
SoundT &sound_;
// TODO: real video output.
- int position_ = 0;
+ uint32_t position_ = 0;
struct Dimension {
uint32_t period = 0;
@@ -166,6 +189,9 @@ private:
};
Dimension horizontal_, vertical_;
bool entered_sync_ = false;
+
+ int sub_clock_ = 0;
+ int clock_divider_ = 0;
};
}