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After a discussion with others on the Vogons forums, it seems both Tandy and PCjr report memory size as real memory size minus 16KB, and it stays that way even if you later use the 32KB 16-color modes. Hope your program can keep the MCBs and important stuff out of the way!

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Jonathan Campbell
2021-03-23 18:37:02 -07:00
parent fd3c872231
commit 3e29c44205
1 changed files with 17 additions and 3 deletions
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20
src/ints/bios.cpp
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@@ -9307,11 +9307,25 @@ public:
if ((t_conv % 32) != 0)
LOG(LOG_MISC,LOG_WARN)("Warning: Conventional memory size %uKB in Tandy mode is not a multiple of 32KB, games may not display graphics correctly",t_conv);
/* take 32KB off the top for video RAM. */
/* Take 16KB off the top for video RAM.
* This value never changes after boot, even if you then use the 16-color modes which then moves
* the video RAM region down 16KB to make a 32KB region. Neither MS-DOS nor INT 10h change this
* top of memory value. I hope your DOS game doesn't put any important structures or MCBs above
* the 32KB below top of memory, because it WILL get overwritten with graphics!
*
* This is apparently correct behavior, and DOSBox SVN and other forks follow it too.
*
* See also: [https://www.vogons.org/viewtopic.php?p=948879#p948879]
* Issue: [https://github.com/joncampbell123/dosbox-x/issues/2380]
*
* Mickeys Space Adventure assumes it can find video RAM by calling INT 12h, subtracting 16KB, and
* converting KB to paragraphs. Note that it calls INT 12h while in CGA mode, and subtracts 16KB
* knowing video memory will extend downward 16KB into a 32KB region when it switches into the
* Tandy/PCjr 16-color mode. */
if (t_conv > 640) t_conv = 640;
if (ulimit > 640) ulimit = 640;
t_conv -= 32;
ulimit -= 32;
t_conv -= 16;
ulimit -= 16;
/* FIXME: INT 10h emulation currently points video RAM at 9800:0000 regardless of
* the amount of conventional memory, which happens to work as long as the