# HG changeset patch # User Paul Boddie # Date 1442699161 -7200 # Node ID 0249c8ecb57ed230c924e5691a8026cf89532e63 # Parent 752c60951649676cda506b1ca3cbea387c09336a Added 625-line, 312/313 scanline notes. diff -r 752c60951649 -r 0249c8ecb57e ULA.txt --- a/ULA.txt Mon Sep 07 23:43:48 2015 +0200 +++ b/ULA.txt Sat Sep 19 23:46:01 2015 +0200 @@ -32,6 +32,11 @@ requires at most 80 bytes of data, and that the ULA is apparently busy for 40 out of 64 microseconds in each scanline. +(In fact, since the ULA is seeking to provide an image for an interlaced +625-line display, there are in fact two "fields" involved, one providing 312 +scanlines and one providing 313 scanlines. See below for a description of the +video system.) + Access to RAM involves accessing four 64Kb dynamic RAM devices (IC4 to IC7, each providing two bits of each byte) using two cycles within the 500ns period of the 2MHz clock to complete each access operation. Since the CPU and ULA @@ -156,11 +161,10 @@ cycle. The "vertical blanking period", meaning the period before picture information -in each field is 25 lines out of 312 (strictly 312.5) and thus lasts for -1.6ms. Of this, 2.5 lines occur before the vsync (field sync) which also lasts -for 2.5 lines. Thus, the first visible scanline on the first field of a frame -occurs half way through the 23rd scanline period measured from the start of -vsync: +in each field is 25 lines out of 312 (or 313) and thus lasts for 1.6ms. Of +this, 2.5 lines occur before the vsync (field sync) which also lasts for 2.5 +lines. Thus, the first visible scanline on the first field of a frame occurs +half way through the 23rd scanline period measured from the start of vsync: 10 20 23 Line in frame: 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 @@ -174,7 +178,7 @@ 310 336 Line in frame: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 - Line from 313: 0 23 + Line from 313: 0 23 4 Line on screen: 88:::::VVVVV:::: 11223344 288 | | |_________________________________________________|