From 8f0b72389f0c4eb6fb7a556dcfd22b4f74e2ffcd Mon Sep 17 00:00:00 2001
From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Fri, 24 May 2019 18:14:57 +0100
Subject: [PATCH] add shadowing description

---
 3d_gpu/6600scoreboard.mdwn | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/3d_gpu/6600scoreboard.mdwn b/3d_gpu/6600scoreboard.mdwn
index e990cbbbb..a65c8198b 100644
--- a/3d_gpu/6600scoreboard.mdwn
+++ b/3d_gpu/6600scoreboard.mdwn
@@ -83,3 +83,18 @@ from hazards in the process.
 
 [[!img dependence_cell_pending.jpg]]
 
+# Shadowing
+
+Shadowing is important as it is the fundamental basis of:
+
+* Precise exceptions
+* Write-after-write hazard avoidance
+* Correct multi-issue instruction sequencing
+* Branch speculation
+
+Modifications to the shadow circuit below allow the shadow flip-flops
+to be automatically reset after a Function Unit "dies".  Without these
+modifications, the shadow unit may spuriously fire on subsequent re-use
+due to some of the latches being left in the previous state.
+
+[[!img shadow.jpg]]
-- 
2.30.2