IBM System/360 Read-Only Storage (ROS)

Extensive microcode allowed the System/360 to provide a compatible instruction architecture and compatible I/O across many models having significantly different performance. Among the original offerings, the Model 30, 40 and 50 all had hardware significantly less powerful than the System/360 instruction architecture and implemented the instructions via microcode.

The microcode was contained in a control store implemented as read-only storage, or ROS (this is IBM's term, today we call it read-only memory, or ROM). Then, as today, ROS is implemented as a two-dimensional array of word-lines, which select the output for a particular address, and the orthogonal bit lines which are the outputs. Since only one word line is active at a time, the output for each bit of selected the ROS word can be defined by either no connection at the intersection of the word-line and bit-line, or by connecting the word line to the bit line through a diode. That is, when a bit-line is asserted, the output of the ROS is defined by the connects or no connects to the bit lines for that particular address. The difference among the many implementations of ROM are primarily in the choice of this connection mechanism. Different types of "diodes" affect the speed, size, and ability to change the ROS.

All except the high-end model of the original introduction used microcode to implement the System/360 architecture. Three different types of ROS were used: Card Capacitor ROS, Transformer ROS, Balanced Capacitor ROS. We have samples of all three of the types. Figure 1 shows a storage card of the Model 30's Card Capacitor ROS. The storage card looks like an IBM punched card except it is made of Mylar and has metal lines etched on it. (Much more about this ROS design can be found in J.W.Haskell,Design of a Printed Card Capacitor Read-Only Store, IBM Journal, March 1966.) Figure 2 is a closeup of the card.

Each card has 12 word lines corresponding to the 12 rows on a standard punched card. The contacts for the word lines are on the right in the picture. Each word line attaches to 60 bit "pads" (again corresponding directly to the punch positions in a standard IBM card). The metal pad can be eliminated by punching that position with a standard card punch. Thus, bits in the microcode word for each word line are defined by the metal pad being present or absent.

The ROS cards are placed in a frame where bit lines run vertically aligned with each of the 60 column. A thin sheet of Mylar goes between the card and the bit lines to form the capacitive coupling between the metal pad and the word line. The cards are mounted on a 20x12" board with four cards on each side. The mechanics of the boards (ground planes, pressure on the card, insulator thickness, etc.) are such that the coupling ratio between a punched and unpunched pad is at least 10:1. The boards are assembled into a module to form the complete ROS. The Model 30 ROS used 43 boards for a capacity of 4032 60-bit words (with one spare board). The access time for this ROS is 0.75 microsecond.

It was relatively easy to change ROS cards in installed systems. This flexibility was important because the Model 30 had an option to emulate the machine instruction set of the very popular IBM 1401. Figure 9 (taken from another source) show IBM Field Engineer changing the ROS for a Model 30.

Figure 3 show the code portion of a module from the Model 40 Transformer ROS. Instead of a capacitor forming the connection between the word line and the bit line, the TROS used a transformer at the intersection. Figure 4 is a closeup of one of the Mylar strips containing two 54-bit ROS words. The word line is etched on the strip and completes a loop from one end back to the same end. The current is routed in one of two ways around a central metal core for each bit. The current direction around the core determines whether it is sensed as a 0 or 1. The current direction is controlled by punches that disconnect one side of the current loop around each bit post. (Much more about this ROS design can be found in D.M.Taub,T he Design of Transformer (Diamond Ring) Read-Only Stores, IBM Journal, September 1964.)

Figures 5 & 6 show the "bit line" sensing mechanism that sits on top of the metal cores for each bit. Each module has 128 tapes, each storing 2 words. A total of 16 modules formed the ROS for the Model 40, for a total of 4K words. The access time was 240ns with a cycle time of 625ns. Figure 8 shows 8 modules of the Model 40 ROS; another 8 modules are behind the visible ones.

Figure 10 is part of the Balanced Capacitor ROS of the IBM System/360 Model 50. It is called balanced because the capacitance load on the word line is the same regardless of the pattern of bits. This is as opposed to the Card Capacitor ROS shown in Figure 1 where the number of 1 bits changes the word line capacitance. The balanced approach allows a faster access time: the timing of the Model 50 was 90 ns access and 200 ns cycle. The disadvantage of the balanced approach is that the bit patterns have to be manufactured into the card, as opposed to the card capacitor approach where new ROS bits can be created on site.

Figure 11 shows a closeup of the card. A ROS bit is represented by two locations along the word lines. There are two "word" line for each bit: one is the drive line to trigger reading. Attached to this line is one "pad" shaped per bit. There are two sense lines for each bit on the sensing plane that the ROS cards are pressed against: one for 1 and one for 0. The position of this bit pad over the two sense lines determines whether it is represents a 1 or 0. The other word line a "balance" line for attaching the complement of the bit flags. For example, compare the first two bits of the top two bit rows in Figure 11. The first two bit pads are the same: the right pad is attached to the drive line. The next bits are different: top row has left pad attached, and bext row has right pad attached.

(Much more about this particular ROS design can be found in S.A.Abbas, A Balanced capacitor Read-Only Storage, IBM Journal, July 1968.)