Talk:Intel 8087: Difference between revisions

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Number of Transistors

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How many transistors in the 8087?

IBM

Should add that the 8087 (and later the 80287) received a significant boost when IBM included coprocessor slots on all its early IBM PC motherboards... AnonMoos (talk) 06:22, 18 March 2008 (UTC)[reply]

Bus access and the 8087

The very first text I ever read about the 8087 architecture, explicitly stated that the 8087 did not directly issue the address onto the bus, but relied on the main CPU to interpret the instruction's addressing requirements and then generate the necessary address and read/write signals on the bus. The main CPU then went idle while the FPU locked the bus, read or wrote the data, and then unlocked the bus.

If someone with low-level 8087 experience could clarify, that would be fantastic. 24.166.235.75 (talk) 07:16, 29 October 2010 (UTC)[reply]

The 8087 was directly connected to the 12 bit address bus and the 16 bit data bus (8 bit data bus when used with an 8088). It was normally idle and its bus lines were high impedance allowing the 8086/8 to control the buses. When an instruction that started '11011' (an 'escape' instruction) was placed on the data bus, the main processor immediately placed its address and data buses in a high state and sent a discrete signal to the 8087 effectively waking it up. It read the instruction and then executed that instruction placing any required adress and data on the buses. Once that instruction had finished, control was handed back to the main processor which then fetched the next instruction. If that was an escape instruction, the above repeated.

You may be a bit confused (and I grant that it is quite easy) by the fact that the 8087 never placed the address on the bus to fetch its instructions. These were placed by the main processor which then recognised instructions intended for (any) co-processor. However the 8087 did place addresses that were required as part of the execution of those instructions.

Later co-processors (80287 onwards - there was no 80187) did indeed rely on the main processor to handle all bus traffic. 86.176.155.137 (talk) 19:12, 13 January 2011 (UTC)[reply]

@@ Line 21: / Line 21: @@
 If someone with low-level 8087 experience could clarify, that would be fantastic.
 [[Special:Contributions/24.166.235.75|24.166.235.75]] ([[User talk:24.166.235.75|talk]]) 07:16, 29 October 2010 (UTC)
+:The 8087 was directly connected to the 12 bit address bus and the 16 bit data bus (8 bit data bus when used with an 8088).  It was normally idle and its bus lines were high impedance allowing the 8086/8 to control the buses.  When an instruction that started '11011' (an 'escape' instruction) was placed on the data bus, the main processor immediately placed its address and data buses in a high state and sent a discrete signal to the 8087 effectively waking it up.  It read the instruction and then executed that instruction placing any required adress and data on the buses.  Once that instruction had finished, control was handed back to the main processor which then fetched the next instruction.  If that was an escape instruction, the above repeated.
+:You may be a bit confused (and I grant that it is quite easy) by the fact that the 8087 never placed the address on the bus to fetch its instructions.  These were placed by the main processor which then recognised instructions intended for (any) co-processor.  However the 8087 did place addresses that were required as part of the execution of those instructions.
+:Later co-processors (80287 onwards - there was no 80187) did indeed rely on the main processor to handle all bus traffic. [[Special:Contributions/86.176.155.137|86.176.155.137]] ([[User talk:86.176.155.137|talk]]) 19:12, 13 January 2011 (UTC)