Browse Prior Art Database

Microcode Transparent Virtual Control Store Addressing

IP.com Disclosure Number: IPCOM000039260D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 60K

Publishing Venue

IBM

Related People

Funk, MR: AUTHOR [+4]

Abstract

The effect of this invention is to allow the determination of whether a normally non-resident module is currently in resident control store, transparent to the microcode. This transparency occurs due to the concept of virtual addressing of normally non-resident control words. The hardware accomplishes this by a method similar to main store paging, where control store modules are analogous to main store pages. (Image Omitted) One consequence is that the microcode modules can be rearranged in different fashions with no concern over branching to virtual addresses. This invention accomplishes the following: Frees the microcode from the extra bookkeeping, control words, and time it takes for the microcode to check whether a virtual address corresponds to a real location in control store.

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Microcode Transparent Virtual Control Store Addressing

The effect of this invention is to allow the determination of whether a normally non-resident module is currently in resident control store, transparent to the microcode. This transparency occurs due to the concept of virtual addressing of normally non-resident control words. The hardware accomplishes this by a method similar to main store paging, where control store modules are analogous to main store pages.

(Image Omitted)

One consequence is that the microcode modules can be rearranged in different fashions with no concern over branching to virtual addresses. This invention accomplishes the following:

Frees the microcode from the extra bookkeeping,

control words, and time it takes for the microcode

to check whether a virtual address corresponds to

a real location in control store.

If a virtual address does correspond to a real

location, the real address is computed and a

branch is done to the proper real location.

If a virtual address does NOT correspond to a real

location, this invention saves the virtual address

requested and branches to a predefined control

store address. The microcode at this address

performs the control store overlay, placing the

code at the virtual address from main store into a

real location in control store.

When the process overlaying control words from

main store into control store is complete,

microcode within the Overlay Handler branches to

the now resident module. The real control store

address used in this branch had been translated by

the Overlay Handler from the virtual address saved

in the step above. Fig. 1 shows how the hardware handles virtual addresses. 8K of control store is partitioned into sections called modules, and each module contains 64 control words. See Fig.
2. Eight of these modules are set aside for use as resident overlay modules. When a microcode module is overlayed into control store, it is placed into one of these resident overlay modules. The processor contains an internal array. Eight byte locations in the internal array correspond to the 8 resident overlay modules. These locations save the identification of the virtual address module which is currently residing in the corresponding resident overlay module. For the following discussion, a quick overview of a particular virtual control store addressing scheme is helpful, as seen in Fig. 3.

The microcode can specify virtual addresses up to

64K. This takes 16 bits (numbered 0:15).

A module is 64 control words. Therefore, the

1

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least significant 6 bits (20:15) of the address

specify the p...