one is the direct access oled display module, the other is indirect access.

Display Module with Cygnal microcontroller hardware interface design
VGS12864E interface connection in two ways: one is the direct access mode, the other is indirect access. Either way, you want to access the module must first read the contents of the status register to determine the "busy" flag can be accessed when not busy. Direct Graphic lcd modules  access method is a module interface as a memory or I / O devices directly  oled display module
 linked to the MCU bus, MCU to access memory or I / O device mode the module operation. Indirect access MCU module is operated via software simulation control timing. Here is indirect access interface design Cygnal C8051F020 MCU and VGS12864E of.
Cygnal C8051F020 is an American company launched a mixed-signal SoC-based 8-bit microcontroller, a highly integrated mixed-signal system-on-chip. It has a 100-pin TQFP package, low power Character LCD Display supply voltage of 2.7 ~ 3.3 V, all I / O, RST, JTAG pins are resistant to 5 V voltage; has high speed pipelined 8051-compatible CIP51 core structure (up to 25 MIPS). The MCU has a total of 64 P0 ~ P7 purpose I / O ports, each port pin can be configured as push-pull output or open-drain output. For VGS12864E, due to its operating voltage is 5 V, and C8051F020 operating voltage is 3.3 V, so the output C8051F020 better able to drive 5 V input OLED, the system requires additional configuration. In addition to the corresponding output port is set to "open drain", but also in the circuit will be connected to each port through a pull-up resistor on a 5 V power supply, so you can ensure C8051F020 logic "1" output can be raised to 5 V. The interface circuit shown in Figure 2.
4 software programming