The main memory or RAM is called Read/Write Storage (short: RWS) by IBM. The RWS is located on module cards M2, M4, N2 and N4, depending on the installed options, in this order. Each card has a capacity of 16kB.

Memory map

Memory data flow

When reading data from RWS the information is gated over the Storage R/W bus to the processor and, if Cycle Steal is active, to the display adapter.
Data to be written into RWS is gated over the same bus from the processor to RWS.

Memory addressing

Memory (RWS or ROS) is addressed with the help of the Storage Address Register (SAR). The meaning of the individual address bits is shown in the following picture:

Meaning of the SAR bits

Be careful that the bits are counted from left to right so that bit 0 is the most significant bit and bit 15 is the least significant bit!
If bits 0 to 8 are all zero the processor registers are addressed instead of RWS.
Bits 0 and 1 select one out of four memory cards, (0 0) selects card 0 or M2, (0 1) select card 1 or M4 and so on. Bits 3 to 5 are taken as chip select signals for the memory chips. The remaining bits address one specific bit of the selected chip.
A detailed wiring can be found in the memory diagram .