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PCjr: Remove minimum 256KB check, allow setting as low as 64KB, although the DOS kernel still requires 128KB minimum. Fix DOS kernel not to create MCB block past video RAM at top of 128KB if memsize is 128KB. This change makes it possible to run Mickeys Space Adventure in PCjr mode if you set memsize=0 and memsizekb=128. Note the game will corrupt itself if memsize=144 to memsize=256 and will not display anything if memsize=640 which is equivalent to DOSBox SVN PCjr emulation

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This commit is contained in:
Jonathan Campbell
2021-03-23 20:05:59 -07:00
parent 6b2f7fa68e
commit a1685c7120
2 changed files with 30 additions and 9 deletions
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23
src/dos/dos_memory.cpp
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@@ -568,19 +568,26 @@ void DOS_SetupMemory(void) {
* DOSBox-X needs to support memsizekb=64 or memsizekb=128,
* and adjust video ram location appropriately. */
if (seg_limit < ((256*1024)/16))
E_Exit("PCjr requires at least 256K");
/* memory from 128k to 640k is available */
mcb_devicedummy.SetPt((uint16_t)0x2000);
mcb_devicedummy.SetPSPSeg(MCB_FREE);
mcb_devicedummy.SetSize(/*0x9FFF*/(seg_limit-1) - 0x2000);
mcb_devicedummy.SetType(0x5a);
/* TODO: Allow memsizekb=64 to emulate a PCjr without the memory extension */
/* NTS: BIOS takes 16KB off the top, therefore the 128-16 KB check */
if (seg_limit < (((128-16)*1024)/16))
E_Exit("PCjr requires at least 128K");
bool extend = seg_limit > ((128*1024)/16);
if (extend) {
/* memory from 128k to 640k is available */
mcb_devicedummy.SetPt((uint16_t)0x2000);
mcb_devicedummy.SetPSPSeg(MCB_FREE);
mcb_devicedummy.SetSize(/*0x9FFF*/(seg_limit-1) - 0x2000);
mcb_devicedummy.SetType(0x5a);
}
/* exclude PCJr graphics region */
mcb_devicedummy.SetPt((uint16_t)0x17ff);
mcb_devicedummy.SetPSPSeg(MCB_DOS);
mcb_devicedummy.SetSize(0x800);
mcb_devicedummy.SetType(0x4d);
mcb_devicedummy.SetType(extend ? 0x4d/*continue*/ : 0x5a/*end*/);
/* memory below 96k */
mcb.SetSize(0x1800 - DOS_MEM_START - (2+mcb_sizes));