cswap.asm XMS handling not 8086-clean

[ecm-pushbx]

While testing my XMS implementation for 8086tiny ecm-pushbx/8086tiny@55bf203 I came upon several different, reproducable errors produced by different versions of FreeCOM. With a hack to make FreeCOM fail its XMS allocation, everything seemed to work and my own program (lDebug symbolic) successfully used the XMS interface.

I eventually chased the error to the handler that saves to XMS then is meant to jump into the resident portion:

freecom/shell/cswap.asm

Line 405 in 274b4a9

		push dx			; save DS of transient portion
		push cs			; save segment of transient portion
		push WORD ret_from_resident
		push cx
		push WORD real_XMSexec
		retf

Push with an immediate is a 186+ instruction, but FreeCOM doesn't check for an 186 machine before using it.This likely wasn't noticed because XMS with actual extended memory requires a 286 at least.

[ecm-pushbx]

I suggest that the NASM directive "cpu 8086" should be used to catch these and possibly additional bugs.

[ecm-pushbx]

I disabled my first workaround and implemented the push imm16 instruction in my fork of 8086tiny. Now the XMS swapping works. Refer to ecm-pushbx/8086tiny@c09ef85

[bartoldeman]

Indeed XMS on a 8086 is nonsense. But forcing cpu 8086 is not a bad idea in other places and there is a better way in any case (a "call far" instruction).

Some code in the cswap.asm is still not 8088-safe but for the above reason I'm not going to fix it (namely that early models did not disable interrupts for one instruction past mov ss, ...)