From: ian <ian>
Date: Tue, 20 Dec 2005 23:17:13 +0000 (+0000)
Subject: some comments about the buffer management
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ijackson/git?a=commitdiff_plain;h=2b871adc25c16c51673daf08e162f7d1b6f26399;p=trains.git

some comments about the buffer management
---

diff --git a/detpic/nmra-stream.asm b/detpic/nmra-stream.asm
index fe3154c..9a8c675 100644
--- a/detpic/nmra-stream.asm
+++ b/detpic/nmra-stream.asm
@@ -24,10 +24,25 @@ clock equ mclock ; this part runs really only on master
 nmractrl	res	1	; state relevant to NMRA control
 transmitbit	equ	7	; bit 7: 0/1 bit currently being transmitted
 nextaction	equ	6	; bit 6: change polarity on next interrupt y/n
+ ; nmractrl is a bitmask, and only the bits above are used
 
 fromserial	res	1	; from-serial buffer location (in buffer bank)
 totrack		res	1	; to-track buffer location (in buffer bank)
 totrackbit	res	1	; bit location of pointer within byte
+ ;
+ ; buffers are each 16 bytes (this is hardwired)
+ ; and there are four of them starting at 0x500, 0x510, 0x520, 0x530.
+ ;
+ ; fromserial and totrack are pointers into the buffer page
+ ;
+ ; fromserial points to the first empty byte, where we will put new
+ ; data provided by the host; this means that (fromserial & 0xf0) is
+ ; the first buffer which fails to contain a complete nmra message
+ ;
+ ; totrack points to the first remaining full byte, which is currently
+ ; being transmitted; if nothing is currently being transmitted
+ ; because there is no nmra data at all then totrack points to the
+ ; start of the same buffer as fromserial
 
 bufferpage	equ	5