In shadow mode the RAM can be thought of as 44K from address &0000- &AFFF, plus a parallel bank of 20K RAM from address &3000-&7FFF which we shall call "shadow RAM". As in normal mode, the top 12K of normal RAM is paged into the memory when required. In the address range &3000-&7FFF the PAL (IC36) is able to switch between shadow RAM and normal RAM. It selects access to the shadow memory if a) shadow mode is on b) it detects a VDU driver and c) the operand address is between &3000 and &7FFF, the part of the memory map used by the screen. Otherwise it selects access to normal RAM.
The machines logic is set to shadow mode when logic 1 is written to D7 at address &FE34, this causes pin 17 (VDUSEL) to go high. &FE34 is the address of a register in the PAL and when D7 is set any screen access through the VDU drivers will cause the PAL to switch in the shadow memory by making pin 12 (CPUSEL) high. In shadow node, VDUSEL is always set, and CPUSEL is low to access normal RAM and high to access shadow RAM. When the paged RAM is selected in shadow mode, the top 4K, &A000 to &AFFF, is programmed by the PAL (IC36) to have the attributes of VDU drivers. Any code executing between &0000-&9FFF in shadow mode will always access normal RAM.
Any code executing from sideways RAM between &A000-&AFFF will access the shadow RAM (if selected) when the operand address is between &3000-&7FFF. This special attribute is not available to any other sideways memory, ROM or RAM. 5.4.4 RAM circuitry The 64K installed RAM is provided by eight 64K by 1 dynamic memory devices ICs 55, 56, 60, 61, 64, 65, 66, and 67. Figure 6 shows the RAM timing diagram. Figure 6 RAM timing diagram 18